chromeos: Convert D-Bus client classes' callback arguments to const reference

BUG=chromium-os:28578
TEST=build success

Review URL: https://chromiumcodereview.appspot.com/10024054

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@131697 0039d316-1c4b-4281-b951-d872f2087c98

24 files changed, 192 insertions, 167 deletions

diff --git a/chromeos/dbus/bluetooth_property.h b/chromeos/dbus/bluetooth_property.h
index ccfbff1..20050c9 100644
--- a/chromeos/dbus/bluetooth_property.h
+++ b/chromeos/dbus/bluetooth_property.h
@@ -23,7 +23,7 @@ class BluetoothPropertySet : public dbus::PropertySet {
  public:
   BluetoothPropertySet(dbus::ObjectProxy* object_proxy,
                        const std::string& interface,
-                       PropertyChangedCallback callback)
+                       const PropertyChangedCallback& callback)
       : dbus::PropertySet(object_proxy, interface, callback) {}
 
   // dbus::PropertySet override.
diff --git a/chromeos/dbus/cashew_client.cc b/chromeos/dbus/cashew_client.cc
index 93d1f3d..eb801f4 100644
--- a/chromeos/dbus/cashew_client.cc
+++ b/chromeos/dbus/cashew_client.cc
@@ -41,7 +41,7 @@ class CashewClientImpl : public CashewClient {
 
   // CashewClient override.
   virtual void SetDataPlansUpdateHandler(
-      DataPlansUpdateHandler handler) OVERRIDE {
+      const DataPlansUpdateHandler& handler) OVERRIDE {
     data_plans_update_handler_ = handler;
   }
 
@@ -101,7 +101,7 @@ class CashewClientStubImpl : public CashewClient {
 
   // CashewClient override.
   virtual void SetDataPlansUpdateHandler(
-      DataPlansUpdateHandler handler) OVERRIDE {}
+      const DataPlansUpdateHandler& handler) OVERRIDE {}
 
   // CashewClient override.
   virtual void ResetDataPlansUpdateHandler() OVERRIDE {}
diff --git a/chromeos/dbus/cashew_client.h b/chromeos/dbus/cashew_client.h
index 2238e1b..195f256 100644
--- a/chromeos/dbus/cashew_client.h
+++ b/chromeos/dbus/cashew_client.h
@@ -44,7 +44,8 @@ class CHROMEOS_EXPORT CashewClient {
                               dbus::Bus* bus);
 
   // Sets DataPlansUpdate signal handler.
-  virtual void SetDataPlansUpdateHandler(DataPlansUpdateHandler handler) = 0;
+  virtual void SetDataPlansUpdateHandler(
+      const DataPlansUpdateHandler& handler) = 0;
 
   // Resets DataPlansUpdate signal handler.
   virtual void ResetDataPlansUpdateHandler() = 0;
diff --git a/chromeos/dbus/cros_disks_client.cc b/chromeos/dbus/cros_disks_client.cc
index 217382c..441b7fe 100644
--- a/chromeos/dbus/cros_disks_client.cc
+++ b/chromeos/dbus/cros_disks_client.cc
@@ -114,8 +114,8 @@ class CrosDisksClientImpl : public CrosDisksClient {
   // CrosDisksClient override.
   virtual void Mount(const std::string& source_path,
                      MountType type,
-                     MountCallback callback,
-                     ErrorCallback error_callback) OVERRIDE {
+                     const MountCallback& callback,
+                     const ErrorCallback& error_callback) OVERRIDE {
     dbus::MethodCall method_call(cros_disks::kCrosDisksInterface,
                                  cros_disks::kMount);
     dbus::MessageWriter writer(&method_call);
@@ -134,8 +134,8 @@ class CrosDisksClientImpl : public CrosDisksClient {
 
   // CrosDisksClient override.
   virtual void Unmount(const std::string& device_path,
-                       UnmountCallback callback,
-                       ErrorCallback error_callback) OVERRIDE {
+                       const UnmountCallback& callback,
+                       const ErrorCallback& error_callback) OVERRIDE {
     dbus::MethodCall method_call(cros_disks::kCrosDisksInterface,
                                  cros_disks::kUnmount);
     dbus::MessageWriter writer(&method_call);
@@ -154,8 +154,8 @@ class CrosDisksClientImpl : public CrosDisksClient {
 
   // CrosDisksClient override.
   virtual void EnumerateAutoMountableDevices(
-      EnumerateAutoMountableDevicesCallback callback,
-      ErrorCallback error_callback) OVERRIDE {
+      const EnumerateAutoMountableDevicesCallback& callback,
+      const ErrorCallback& error_callback) OVERRIDE {
     dbus::MethodCall method_call(cros_disks::kCrosDisksInterface,
                                  cros_disks::kEnumerateAutoMountableDevices);
     proxy_->CallMethod(
@@ -169,8 +169,8 @@ class CrosDisksClientImpl : public CrosDisksClient {
   // CrosDisksClient override.
   virtual void FormatDevice(const std::string& device_path,
                             const std::string& filesystem,
-                            FormatDeviceCallback callback,
-                            ErrorCallback error_callback) OVERRIDE {
+                            const FormatDeviceCallback& callback,
+                            const ErrorCallback& error_callback) OVERRIDE {
     dbus::MethodCall method_call(cros_disks::kCrosDisksInterface,
                                  cros_disks::kFormatDevice);
     dbus::MessageWriter writer(&method_call);
@@ -185,9 +185,10 @@ class CrosDisksClientImpl : public CrosDisksClient {
   }
 
   // CrosDisksClient override.
-  virtual void GetDeviceProperties(const std::string& device_path,
-                                   GetDevicePropertiesCallback callback,
-                                   ErrorCallback error_callback) OVERRIDE {
+  virtual void GetDeviceProperties(
+      const std::string& device_path,
+      const GetDevicePropertiesCallback& callback,
+      const ErrorCallback& error_callback) OVERRIDE {
     dbus::MethodCall method_call(cros_disks::kCrosDisksInterface,
                                  cros_disks::kGetDeviceProperties);
     dbus::MessageWriter writer(&method_call);
@@ -203,8 +204,8 @@ class CrosDisksClientImpl : public CrosDisksClient {
 
   // CrosDisksClient override.
   virtual void SetUpConnections(
-      MountEventHandler mount_event_handler,
-      MountCompletedHandler mount_completed_handler) OVERRIDE {
+      const MountEventHandler& mount_event_handler,
+      const MountCompletedHandler& mount_completed_handler) OVERRIDE {
     static const SignalEventTuple kSignalEventTuples[] = {
       { cros_disks::kDeviceAdded, DEVICE_ADDED },
       { cros_disks::kDeviceScanned, DEVICE_SCANNED },
@@ -246,8 +247,8 @@ class CrosDisksClientImpl : public CrosDisksClient {
   };
 
   // Handles the result of Mount and calls |callback| or |error_callback|.
-  void OnMount(MountCallback callback,
-               ErrorCallback error_callback,
+  void OnMount(const MountCallback& callback,
+               const ErrorCallback& error_callback,
                dbus::Response* response) {
     if (!response) {
       error_callback.Run();
@@ -258,8 +259,8 @@ class CrosDisksClientImpl : public CrosDisksClient {
 
   // Handles the result of Unount and calls |callback| or |error_callback|.
   void OnUnmount(const std::string& device_path,
-                 UnmountCallback callback,
-                 ErrorCallback error_callback,
+                 const UnmountCallback& callback,
+                 const ErrorCallback& error_callback,
                  dbus::Response* response) {
     if (!response) {
       error_callback.Run();
@@ -271,8 +272,8 @@ class CrosDisksClientImpl : public CrosDisksClient {
   // Handles the result of EnumerateAutoMountableDevices and calls |callback| or
   // |error_callback|.
   void OnEnumerateAutoMountableDevices(
-      EnumerateAutoMountableDevicesCallback callback,
-      ErrorCallback error_callback,
+      const EnumerateAutoMountableDevicesCallback& callback,
+      const ErrorCallback& error_callback,
       dbus::Response* response) {
     if (!response) {
       error_callback.Run();
@@ -291,8 +292,8 @@ class CrosDisksClientImpl : public CrosDisksClient {
   // Handles the result of FormatDevice and calls |callback| or
   // |error_callback|.
   void OnFormatDevice(const std::string& device_path,
-                      FormatDeviceCallback callback,
-                      ErrorCallback error_callback,
+                      const FormatDeviceCallback& callback,
+                      const ErrorCallback& error_callback,
                       dbus::Response* response) {
     if (!response) {
       error_callback.Run();
@@ -311,8 +312,8 @@ class CrosDisksClientImpl : public CrosDisksClient {
   // Handles the result of GetDeviceProperties and calls |callback| or
   // |error_callback|.
   void OnGetDeviceProperties(const std::string& device_path,
-                             GetDevicePropertiesCallback callback,
-                             ErrorCallback error_callback,
+                             const GetDevicePropertiesCallback& callback,
+                             const ErrorCallback& error_callback,
                              dbus::Response* response) {
     if (!response) {
       error_callback.Run();
@@ -375,24 +376,25 @@ class CrosDisksClientStubImpl : public CrosDisksClient {
 
   virtual void Mount(const std::string& source_path,
                      MountType type,
-                     MountCallback callback,
-                     ErrorCallback error_callback) OVERRIDE {}
+                     const MountCallback& callback,
+                     const ErrorCallback& error_callback) OVERRIDE {}
   virtual void Unmount(const std::string& device_path,
-                       UnmountCallback callback,
-                       ErrorCallback error_callback) OVERRIDE {}
+                       const UnmountCallback& callback,
+                       const ErrorCallback& error_callback) OVERRIDE {}
   virtual void EnumerateAutoMountableDevices(
-      EnumerateAutoMountableDevicesCallback callback,
-      ErrorCallback error_callback) OVERRIDE {}
+      const EnumerateAutoMountableDevicesCallback& callback,
+      const ErrorCallback& error_callback) OVERRIDE {}
   virtual void FormatDevice(const std::string& device_path,
                             const std::string& filesystem,
-                            FormatDeviceCallback callback,
-                            ErrorCallback error_callback) OVERRIDE {}
-  virtual void GetDeviceProperties(const std::string& device_path,
-                                   GetDevicePropertiesCallback callback,
-                                   ErrorCallback error_callback) OVERRIDE {}
+                            const FormatDeviceCallback& callback,
+                            const ErrorCallback& error_callback) OVERRIDE {}
+  virtual void GetDeviceProperties(
+      const std::string& device_path,
+      const GetDevicePropertiesCallback& callback,
+      const ErrorCallback& error_callback) OVERRIDE {}
   virtual void SetUpConnections(
-      MountEventHandler mount_event_handler,
-      MountCompletedHandler mount_completed_handler) OVERRIDE {}
+      const MountEventHandler& mount_event_handler,
+      const MountCompletedHandler& mount_completed_handler) OVERRIDE {}
 
  private:
   DISALLOW_COPY_AND_ASSIGN(CrosDisksClientStubImpl);
diff --git a/chromeos/dbus/cros_disks_client.h b/chromeos/dbus/cros_disks_client.h
index df7cf49..88e6aee 100644
--- a/chromeos/dbus/cros_disks_client.h
+++ b/chromeos/dbus/cros_disks_client.h
@@ -180,40 +180,40 @@ class CHROMEOS_EXPORT CrosDisksClient {
   // otherwise, |error_callback| is called.
   virtual void Mount(const std::string& source_path,
                      MountType type,
-                     MountCallback callback,
-                     ErrorCallback error_callback) = 0;
+                     const MountCallback& callback,
+                     const ErrorCallback& error_callback) = 0;
 
   // Calls Unmount method.  |callback| is called after the method call succeeds,
   // otherwise, |error_callback| is called.
   virtual void Unmount(const std::string& device_path,
-                       UnmountCallback callback,
-                       ErrorCallback error_callback) = 0;
+                       const UnmountCallback& callback,
+                       const ErrorCallback& error_callback) = 0;
 
   // Calls EnumerateAutoMountableDevices method.  |callback| is called after the
   // method call succeeds, otherwise, |error_callback| is called.
   virtual void EnumerateAutoMountableDevices(
-      EnumerateAutoMountableDevicesCallback callback,
-      ErrorCallback error_callback) = 0;
+      const EnumerateAutoMountableDevicesCallback& callback,
+      const ErrorCallback& error_callback) = 0;
 
   // Calls FormatDevice method.  |callback| is called after the method call
   // succeeds, otherwise, |error_callback| is called.
   virtual void FormatDevice(const std::string& device_path,
                             const std::string& filesystem,
-                            FormatDeviceCallback callback,
-                            ErrorCallback error_callback) = 0;
+                            const FormatDeviceCallback& callback,
+                            const ErrorCallback& error_callback) = 0;
 
   // Calls GetDeviceProperties method.  |callback| is called after the method
   // call succeeds, otherwise, |error_callback| is called.
   virtual void GetDeviceProperties(const std::string& device_path,
-                                   GetDevicePropertiesCallback callback,
-                                   ErrorCallback error_callback) = 0;
+                                   const GetDevicePropertiesCallback& callback,
+                                   const ErrorCallback& error_callback) = 0;
 
   // Registers given callback for events.
   // |mount_event_handler| is called when mount event signal is received.
   // |mount_completed_handler| is called when MountCompleted signal is received.
   virtual void SetUpConnections(
-      MountEventHandler mount_event_handler,
-      MountCompletedHandler mount_completed_handler) = 0;
+      const MountEventHandler& mount_event_handler,
+      const MountCompletedHandler& mount_completed_handler) = 0;
 
   // Factory function, creates a new instance and returns ownership.
   // For normal usage, access the singleton via DBusThreadManager::Get().
diff --git a/chromeos/dbus/cryptohome_client.cc b/chromeos/dbus/cryptohome_client.cc
index b4d9e3f..14fa92b1 100644
--- a/chromeos/dbus/cryptohome_client.cc
+++ b/chromeos/dbus/cryptohome_client.cc
@@ -42,7 +42,7 @@ class CryptohomeClientImpl : public CryptohomeClient {
   }
 
   // CryptohomeClient override.
-  virtual void SetAsyncCallStatusHandler(AsyncCallStatusHandler handler)
+  virtual void SetAsyncCallStatusHandler(const AsyncCallStatusHandler& handler)
       OVERRIDE {
     async_call_status_handler_ = handler;
   }
@@ -67,7 +67,7 @@ class CryptohomeClientImpl : public CryptohomeClient {
   // CryptohomeClient override.
   virtual void AsyncCheckKey(const std::string& username,
                              const std::string& key,
-                             AsyncMethodCallback callback) OVERRIDE {
+                             const AsyncMethodCallback& callback) OVERRIDE {
     INITIALIZE_METHOD_CALL(method_call, cryptohome::kCryptohomeAsyncCheckKey);
     dbus::MessageWriter writer(&method_call);
     writer.AppendString(username);
@@ -82,7 +82,7 @@ class CryptohomeClientImpl : public CryptohomeClient {
   virtual void AsyncMigrateKey(const std::string& username,
                                const std::string& from_key,
                                const std::string& to_key,
-                               AsyncMethodCallback callback) OVERRIDE {
+                               const AsyncMethodCallback& callback) OVERRIDE {
     INITIALIZE_METHOD_CALL(method_call, cryptohome::kCryptohomeAsyncMigrateKey);
     dbus::MessageWriter writer(&method_call);
     writer.AppendString(username);
@@ -96,7 +96,7 @@ class CryptohomeClientImpl : public CryptohomeClient {
 
   // CryptohomeClient override.
   virtual void AsyncRemove(const std::string& username,
-                           AsyncMethodCallback callback) OVERRIDE {
+                           const AsyncMethodCallback& callback) OVERRIDE {
     INITIALIZE_METHOD_CALL(method_call, cryptohome::kCryptohomeAsyncRemove);
     dbus::MessageWriter writer(&method_call);
     writer.AppendString(username);
@@ -126,7 +126,7 @@ class CryptohomeClientImpl : public CryptohomeClient {
   virtual void AsyncMount(const std::string& username,
                           const std::string& key,
                           const bool create_if_missing,
-                          AsyncMethodCallback callback) OVERRIDE {
+                          const AsyncMethodCallback& callback) OVERRIDE {
     INITIALIZE_METHOD_CALL(method_call, cryptohome::kCryptohomeAsyncMount);
     dbus::MessageWriter writer(&method_call);
     writer.AppendString(username);
@@ -142,7 +142,7 @@ class CryptohomeClientImpl : public CryptohomeClient {
   }
 
   // CryptohomeClient override.
-  virtual void AsyncMountGuest(AsyncMethodCallback callback) OVERRIDE {
+  virtual void AsyncMountGuest(const AsyncMethodCallback& callback) OVERRIDE {
     INITIALIZE_METHOD_CALL(method_call, cryptohome::kCryptohomeAsyncMountGuest);
     proxy_->CallMethod(&method_call, dbus::ObjectProxy::TIMEOUT_USE_DEFAULT,
                        base::Bind(&CryptohomeClientImpl::OnAsyncMethodCall,
@@ -157,7 +157,7 @@ class CryptohomeClientImpl : public CryptohomeClient {
   }
 
   // CryptohomeClient override.
-  virtual void TpmIsEnabled(BoolMethodCallback callback) OVERRIDE {
+  virtual void TpmIsEnabled(const BoolMethodCallback& callback) OVERRIDE {
     INITIALIZE_METHOD_CALL(method_call, cryptohome::kCryptohomeTpmIsEnabled);
     proxy_->CallMethod(
         &method_call, dbus::ObjectProxy::TIMEOUT_USE_DEFAULT,
@@ -219,7 +219,7 @@ class CryptohomeClientImpl : public CryptohomeClient {
   }
 
   // CryptohomeClient override.
-  virtual void Pkcs11IsTpmTokenReady(BoolMethodCallback callback)
+  virtual void Pkcs11IsTpmTokenReady(const BoolMethodCallback& callback)
       OVERRIDE {
     INITIALIZE_METHOD_CALL(method_call,
                            cryptohome::kCryptohomePkcs11IsTpmTokenReady);
@@ -232,8 +232,8 @@ class CryptohomeClientImpl : public CryptohomeClient {
   }
 
   // CryptohomeClient override.
-  virtual void Pkcs11GetTpmTokenInfo(Pkcs11GetTpmTokenInfoCallback callback)
-      OVERRIDE {
+  virtual void Pkcs11GetTpmTokenInfo(
+      const Pkcs11GetTpmTokenInfoCallback& callback) OVERRIDE {
     INITIALIZE_METHOD_CALL(method_call,
                            cryptohome::kCryptohomePkcs11GetTpmTokenInfo);
     proxy_->CallMethod(
@@ -309,7 +309,7 @@ class CryptohomeClientImpl : public CryptohomeClient {
 
  private:
   // Handles the result of AsyncXXX methods.
-  void OnAsyncMethodCall(AsyncMethodCallback callback,
+  void OnAsyncMethodCall(const AsyncMethodCallback& callback,
                          dbus::Response* response) {
     if (!response)
       return;
@@ -334,7 +334,7 @@ class CryptohomeClientImpl : public CryptohomeClient {
   }
 
   // Handles responses for methods with a bool value result.
-  void OnBoolMethod(BoolMethodCallback callback,
+  void OnBoolMethod(const BoolMethodCallback& callback,
                     dbus::Response* response) {
     if (!response) {
       callback.Run(DBUS_METHOD_CALL_FAILURE, false);
@@ -350,7 +350,7 @@ class CryptohomeClientImpl : public CryptohomeClient {
   }
 
   // Handles responses for Pkcs11GetTpmtTokenInfo.
-  void OnPkcs11GetTpmTokenInfo(Pkcs11GetTpmTokenInfoCallback callback,
+  void OnPkcs11GetTpmTokenInfo(const Pkcs11GetTpmTokenInfoCallback& callback,
                                dbus::Response* response) {
     if (!response) {
       callback.Run(DBUS_METHOD_CALL_FAILURE, std::string(), std::string());
@@ -411,7 +411,7 @@ class CryptohomeClientStubImpl : public CryptohomeClient {
   virtual ~CryptohomeClientStubImpl() {}
 
   // CryptohomeClient override.
-  virtual void SetAsyncCallStatusHandler(AsyncCallStatusHandler handler)
+  virtual void SetAsyncCallStatusHandler(const AsyncCallStatusHandler& handler)
       OVERRIDE {
     async_call_status_handler_ = handler;
   }
@@ -436,7 +436,7 @@ class CryptohomeClientStubImpl : public CryptohomeClient {
   // CryptohomeClient override.
   virtual void AsyncCheckKey(const std::string& username,
                              const std::string& key,
-                             AsyncMethodCallback callback) OVERRIDE {
+                             const AsyncMethodCallback& callback) OVERRIDE {
     ReturnAsyncMethodResult(callback);
   }
 
@@ -444,13 +444,13 @@ class CryptohomeClientStubImpl : public CryptohomeClient {
   virtual void AsyncMigrateKey(const std::string& username,
                                const std::string& from_key,
                                const std::string& to_key,
-                               AsyncMethodCallback callback) OVERRIDE {
+                               const AsyncMethodCallback& callback) OVERRIDE {
     ReturnAsyncMethodResult(callback);
   }
 
   // CryptohomeClient override.
   virtual void AsyncRemove(const std::string& username,
-                           AsyncMethodCallback callback) OVERRIDE {
+                           const AsyncMethodCallback& callback) OVERRIDE {
     ReturnAsyncMethodResult(callback);
   }
 
@@ -466,12 +466,12 @@ class CryptohomeClientStubImpl : public CryptohomeClient {
   virtual void AsyncMount(const std::string& username,
                           const std::string& key,
                           const bool create_if_missing,
-                          AsyncMethodCallback callback) OVERRIDE {
+                          const AsyncMethodCallback& callback) OVERRIDE {
     ReturnAsyncMethodResult(callback);
   }
 
   // CryptohomeClient override.
-  virtual void AsyncMountGuest(AsyncMethodCallback callback) OVERRIDE {
+  virtual void AsyncMountGuest(const AsyncMethodCallback& callback) OVERRIDE {
     ReturnAsyncMethodResult(callback);
   }
 
@@ -482,7 +482,7 @@ class CryptohomeClientStubImpl : public CryptohomeClient {
   }
 
   // CryptohomeClient override.
-  virtual void TpmIsEnabled(BoolMethodCallback callback) OVERRIDE {
+  virtual void TpmIsEnabled(const BoolMethodCallback& callback) OVERRIDE {
     MessageLoop::current()->PostTask(
         FROM_HERE, base::Bind(callback, DBUS_METHOD_CALL_SUCCESS, true));
   }
@@ -519,14 +519,15 @@ class CryptohomeClientStubImpl : public CryptohomeClient {
   virtual bool TpmClearStoredPassword() OVERRIDE { return true; }
 
   // CryptohomeClient override.
-  virtual void Pkcs11IsTpmTokenReady(BoolMethodCallback callback) OVERRIDE {
+  virtual void Pkcs11IsTpmTokenReady(
+      const BoolMethodCallback& callback) OVERRIDE {
     MessageLoop::current()->PostTask(
         FROM_HERE, base::Bind(callback, DBUS_METHOD_CALL_SUCCESS, true));
   }
 
   // CryptohomeClient override.
   virtual void Pkcs11GetTpmTokenInfo(
-      Pkcs11GetTpmTokenInfoCallback callback) OVERRIDE {
+      const Pkcs11GetTpmTokenInfoCallback& callback) OVERRIDE {
     const char kStubLabel[] = "Stub TPM Token";
     const char kStubUserPin[] = "012345";
     MessageLoop::current()->PostTask(
@@ -585,7 +586,7 @@ class CryptohomeClientStubImpl : public CryptohomeClient {
 
  private:
   // Posts tasks which return fake results to the UI thread.
-  void ReturnAsyncMethodResult(AsyncMethodCallback callback) {
+  void ReturnAsyncMethodResult(const AsyncMethodCallback& callback) {
     MessageLoop::current()->PostTask(
         FROM_HERE,
         base::Bind(&CryptohomeClientStubImpl::ReturnAsyncMethodResultInternal,
@@ -594,7 +595,7 @@ class CryptohomeClientStubImpl : public CryptohomeClient {
   }
 
   // This method is used to implement ReturnAsyncMethodResult.
-  void ReturnAsyncMethodResultInternal(AsyncMethodCallback callback) {
+  void ReturnAsyncMethodResultInternal(const AsyncMethodCallback& callback) {
     callback.Run(async_call_id_);
     if (!async_call_status_handler_.is_null()) {
       MessageLoop::current()->PostTask(
diff --git a/chromeos/dbus/cryptohome_client.h b/chromeos/dbus/cryptohome_client.h
index df93832..a152e44 100644
--- a/chromeos/dbus/cryptohome_client.h
+++ b/chromeos/dbus/cryptohome_client.h
@@ -51,7 +51,8 @@ class CHROMEOS_EXPORT CryptohomeClient {
   // |handler| is called when results for AsyncXXX methods are returned.
   // Cryptohome service will process the calls in a first-in-first-out manner
   // when they are made in parallel.
-  virtual void SetAsyncCallStatusHandler(AsyncCallStatusHandler handler) = 0;
+  virtual void SetAsyncCallStatusHandler(
+      const AsyncCallStatusHandler& handler) = 0;
 
   // Resets AsyncCallStatus signal handler.
   virtual void ResetAsyncCallStatusHandler() = 0;
@@ -68,19 +69,19 @@ class CHROMEOS_EXPORT CryptohomeClient {
   // succeeds.
   virtual void AsyncCheckKey(const std::string& username,
                              const std::string& key,
-                             AsyncMethodCallback callback) = 0;
+                             const AsyncMethodCallback& callback) = 0;
 
   // Calls AsyncMigrateKey method.  |callback| is called after the method call
   // succeeds.
   virtual void AsyncMigrateKey(const std::string& username,
                                const std::string& from_key,
                                const std::string& to_key,
-                               AsyncMethodCallback callback) = 0;
+                               const AsyncMethodCallback& callback) = 0;
 
   // Calls AsyncRemove method.  |callback| is called after the method call
   // succeeds.
   virtual void AsyncRemove(const std::string& username,
-                           AsyncMethodCallback callback) = 0;
+                           const AsyncMethodCallback& callback) = 0;
 
   // Calls GetSystemSalt method.  This method blocks until the call returns.
   // The original content of |salt| is lost.
@@ -91,18 +92,18 @@ class CHROMEOS_EXPORT CryptohomeClient {
   virtual void AsyncMount(const std::string& username,
                           const std::string& key,
                           const bool create_if_missing,
-                          AsyncMethodCallback callback) = 0;
+                          const AsyncMethodCallback& callback) = 0;
 
   // Calls AsyncMountGuest method.  |callback| is called after the method call
   // succeeds.
-  virtual void AsyncMountGuest(AsyncMethodCallback callback) = 0;
+  virtual void AsyncMountGuest(const AsyncMethodCallback& callback) = 0;
 
   // Calls TpmIsReady method and returns true when the call succeeds.
   // This method blocks until the call returns.
   virtual bool TpmIsReady(bool* ready) = 0;
 
   // Calls TpmIsEnabled method.
-  virtual void TpmIsEnabled(BoolMethodCallback callback) = 0;
+  virtual void TpmIsEnabled(const BoolMethodCallback& callback) = 0;
 
   // Calls TpmIsEnabled method and returns true when the call succeeds.
   // This method blocks until the call returns.
@@ -131,11 +132,11 @@ class CHROMEOS_EXPORT CryptohomeClient {
   virtual bool TpmClearStoredPassword() = 0;
 
   // Calls Pkcs11IsTpmTokenReady method.
-  virtual void Pkcs11IsTpmTokenReady(BoolMethodCallback callback) = 0;
+  virtual void Pkcs11IsTpmTokenReady(const BoolMethodCallback& callback) = 0;
 
   // Calls Pkcs11GetTpmTokenInfo method.
   virtual void Pkcs11GetTpmTokenInfo(
-      Pkcs11GetTpmTokenInfoCallback callback) = 0;
+      const Pkcs11GetTpmTokenInfoCallback& callback) = 0;
 
   // Calls InstallAttributesGet method and returns true when the call succeeds.
   // This method blocks until the call returns.
diff --git a/chromeos/dbus/flimflam_client_unittest_base.cc b/chromeos/dbus/flimflam_client_unittest_base.cc
index d9591f8..62bdeab 100644
--- a/chromeos/dbus/flimflam_client_unittest_base.cc
+++ b/chromeos/dbus/flimflam_client_unittest_base.cc
@@ -67,7 +67,7 @@ void FlimflamClientUnittestBase::TearDown() {
 
 void FlimflamClientUnittestBase::PrepareForMethodCall(
     const std::string& method_name,
-    ArgumentCheckCallback argument_checker,
+    const ArgumentCheckCallback& argument_checker,
     dbus::Response* response) {
   expected_method_name_ = method_name;
   argument_checker_ = argument_checker;
@@ -147,8 +147,8 @@ void FlimflamClientUnittestBase::ExpectDictionaryValueResult(
 void FlimflamClientUnittestBase::OnConnectToSignal(
     const std::string& interface_name,
     const std::string& signal_name,
-    dbus::ObjectProxy::SignalCallback signal_callback,
-    dbus::ObjectProxy::OnConnectedCallback on_connected_callback) {
+    const dbus::ObjectProxy::SignalCallback& signal_callback,
+    const dbus::ObjectProxy::OnConnectedCallback& on_connected_callback) {
   property_changed_handler_ = signal_callback;
   const bool success = true;
   message_loop_.PostTask(FROM_HERE,
@@ -161,7 +161,7 @@ void FlimflamClientUnittestBase::OnConnectToSignal(
 void FlimflamClientUnittestBase::OnCallMethod(
     dbus::MethodCall* method_call,
     int timeout_ms,
-    dbus::ObjectProxy::ResponseCallback response_callback) {
+    const dbus::ObjectProxy::ResponseCallback& response_callback) {
   EXPECT_EQ(interface_name_, method_call->GetInterface());
   EXPECT_EQ(expected_method_name_, method_call->GetMember());
   dbus::MessageReader reader(method_call);
diff --git a/chromeos/dbus/flimflam_client_unittest_base.h b/chromeos/dbus/flimflam_client_unittest_base.h
index 9781fda..3c88b27 100644
--- a/chromeos/dbus/flimflam_client_unittest_base.h
+++ b/chromeos/dbus/flimflam_client_unittest_base.h
@@ -47,7 +47,7 @@ class FlimflamClientUnittestBase : public testing::Test {
 
   // Sets expectations for called method name and arguments, and sets response.
   void PrepareForMethodCall(const std::string& method_name,
-                            ArgumentCheckCallback argument_checker,
+                            const ArgumentCheckCallback& argument_checker,
                             dbus::Response* response);
 
   // Sends property changed signal to the tested client.
@@ -95,14 +95,15 @@ class FlimflamClientUnittestBase : public testing::Test {
   void OnConnectToSignal(
       const std::string& interface_name,
       const std::string& signal_name,
-      dbus::ObjectProxy::SignalCallback signal_callback,
-      dbus::ObjectProxy::OnConnectedCallback on_connected_callback);
+      const dbus::ObjectProxy::SignalCallback& signal_callback,
+      const dbus::ObjectProxy::OnConnectedCallback& on_connected_callback);
 
   // Checks the content of the method call and returns the response.
   // Used to implement the mock proxy.
-  void OnCallMethod(dbus::MethodCall* method_call,
-                    int timeout_ms,
-                    dbus::ObjectProxy::ResponseCallback response_callback);
+  void OnCallMethod(
+      dbus::MethodCall* method_call,
+      int timeout_ms,
+      const dbus::ObjectProxy::ResponseCallback& response_callback);
 
   // The interface name.
   const std::string interface_name_;
diff --git a/chromeos/dbus/image_burner_client.cc b/chromeos/dbus/image_burner_client.cc
index 343c02d..4119bf0 100644
--- a/chromeos/dbus/image_burner_client.cc
+++ b/chromeos/dbus/image_burner_client.cc
@@ -45,7 +45,7 @@ class ImageBurnerClientImpl : public ImageBurnerClient {
   // ImageBurnerClient override.
   virtual void BurnImage(const std::string& from_path,
                          const std::string& to_path,
-                         ErrorCallback error_callback) OVERRIDE {
+                         const ErrorCallback& error_callback) OVERRIDE {
     dbus::MethodCall method_call(imageburn::kImageBurnServiceInterface,
                                  imageburn::kBurnImage);
     dbus::MessageWriter writer(&method_call);
@@ -59,8 +59,8 @@ class ImageBurnerClientImpl : public ImageBurnerClient {
 
   // ImageBurnerClient override.
   virtual void SetEventHandlers(
-      BurnFinishedHandler burn_finished_handler,
-      BurnProgressUpdateHandler burn_progress_update_handler) OVERRIDE {
+      const BurnFinishedHandler& burn_finished_handler,
+      const BurnProgressUpdateHandler& burn_progress_update_handler) OVERRIDE {
     burn_finished_handler_ = burn_finished_handler;
     burn_progress_update_handler_ = burn_progress_update_handler;
   }
@@ -136,10 +136,10 @@ class ImageBurnerClientStubImpl : public ImageBurnerClient {
   virtual ~ImageBurnerClientStubImpl() {}
   virtual void BurnImage(const std::string& from_path,
                          const std::string& to_path,
-                         ErrorCallback error_callback) OVERRIDE {}
+                         const ErrorCallback& error_callback) OVERRIDE {}
   virtual void SetEventHandlers(
-      BurnFinishedHandler burn_finished_handler,
-      BurnProgressUpdateHandler burn_progress_update_handler) OVERRIDE {}
+      const BurnFinishedHandler& burn_finished_handler,
+      const BurnProgressUpdateHandler& burn_progress_update_handler) OVERRIDE {}
   virtual void ResetEventHandlers() OVERRIDE {}
 
  private:
diff --git a/chromeos/dbus/image_burner_client.h b/chromeos/dbus/image_burner_client.h
index 513496d..5368eb0 100644
--- a/chromeos/dbus/image_burner_client.h
+++ b/chromeos/dbus/image_burner_client.h
@@ -42,15 +42,15 @@ class CHROMEOS_EXPORT ImageBurnerClient {
   // Burns the image |from_path| to the disk |to_path|.
   virtual void BurnImage(const std::string& from_path,
                          const std::string& to_path,
-                         ErrorCallback error_callback) = 0;
+                         const ErrorCallback& error_callback) = 0;
 
   // Sets callbacks as event handlers.
   // |burn_finished_handler| is called when burn_finished signal is received.
   // |burn_progress_update_handler| is called when burn_progress_update signal
   // is received.
   virtual void SetEventHandlers(
-      BurnFinishedHandler burn_finished_handler,
-      BurnProgressUpdateHandler burn_progress_update_handler) = 0;
+      const BurnFinishedHandler& burn_finished_handler,
+      const BurnProgressUpdateHandler& burn_progress_update_handler) = 0;
 
   // Resets event handlers. After calling this method, nothing is done when
   // signals are received.
diff --git a/chromeos/dbus/mock_cashew_client.h b/chromeos/dbus/mock_cashew_client.h
index c4167dc..fcff9a9 100644
--- a/chromeos/dbus/mock_cashew_client.h
+++ b/chromeos/dbus/mock_cashew_client.h
@@ -16,7 +16,8 @@ class MockCashewClient : public CashewClient {
   MockCashewClient();
   virtual ~MockCashewClient();
 
-  MOCK_METHOD1(SetDataPlansUpdateHandler, void(DataPlansUpdateHandler handler));
+  MOCK_METHOD1(SetDataPlansUpdateHandler,
+               void(const DataPlansUpdateHandler& handler));
   MOCK_METHOD0(ResetDataPlansUpdateHandler, void());
   MOCK_METHOD0(RequestDataPlansUpdate, void());
 };
diff --git a/chromeos/dbus/mock_cros_disks_client.h b/chromeos/dbus/mock_cros_disks_client.h
index 0b242f98..b258935 100644
--- a/chromeos/dbus/mock_cros_disks_client.h
+++ b/chromeos/dbus/mock_cros_disks_client.h
@@ -18,18 +18,25 @@ class MockCrosDisksClient : public CrosDisksClient {
   MockCrosDisksClient();
   virtual ~MockCrosDisksClient();
 
-  MOCK_METHOD4(Mount, void(const std::string&, MountType, MountCallback,
-                           ErrorCallback));
-  MOCK_METHOD3(Unmount, void(const std::string&, UnmountCallback,
-                             ErrorCallback));
-  MOCK_METHOD2(EnumerateAutoMountableDevices, void(
-      EnumerateAutoMountableDevicesCallback, ErrorCallback));
-  MOCK_METHOD4(FormatDevice, void(const std::string&, const std::string&,
-                                  FormatDeviceCallback, ErrorCallback));
-  MOCK_METHOD3(GetDeviceProperties, void(
-      const std::string&, GetDevicePropertiesCallback, ErrorCallback));
-  MOCK_METHOD2(SetUpConnections, void(MountEventHandler,
-                                      MountCompletedHandler));
+  MOCK_METHOD4(Mount, void(const std::string&,
+                           MountType,
+                           const MountCallback&,
+                           const ErrorCallback&));
+  MOCK_METHOD3(Unmount, void(const std::string&,
+                             const UnmountCallback&,
+                             const ErrorCallback&));
+  MOCK_METHOD2(EnumerateAutoMountableDevices,
+               void(const EnumerateAutoMountableDevicesCallback&,
+                    const ErrorCallback&));
+  MOCK_METHOD4(FormatDevice, void(const std::string&,
+                                  const std::string&,
+                                  const FormatDeviceCallback&,
+                                  const ErrorCallback&));
+  MOCK_METHOD3(GetDeviceProperties, void(const std::string&,
+                                         const GetDevicePropertiesCallback&,
+                                         const ErrorCallback&));
+  MOCK_METHOD2(SetUpConnections, void(const MountEventHandler&,
+                                      const MountCompletedHandler&));
 };
 
 }  // namespace chromeos
diff --git a/chromeos/dbus/mock_cryptohome_client.h b/chromeos/dbus/mock_cryptohome_client.h
index ad6c327..ff5f0b1 100644
--- a/chromeos/dbus/mock_cryptohome_client.h
+++ b/chromeos/dbus/mock_cryptohome_client.h
@@ -18,39 +18,40 @@ class MockCryptohomeClient : public CryptohomeClient {
   MockCryptohomeClient();
   virtual ~MockCryptohomeClient();
 
-  MOCK_METHOD1(SetAsyncCallStatusHandler, void(AsyncCallStatusHandler handler));
+  MOCK_METHOD1(SetAsyncCallStatusHandler,
+               void(const AsyncCallStatusHandler& handler));
   MOCK_METHOD0(ResetAsyncCallStatusHandler, void());
   MOCK_METHOD1(IsMounted, bool(bool* is_mounted));
   MOCK_METHOD1(Unmount, bool(bool* success));
   MOCK_METHOD3(AsyncCheckKey,
                void(const std::string& username,
                     const std::string& key,
-                    base::Callback<void(int async_id)> callback));
+                    const AsyncMethodCallback& callback));
   MOCK_METHOD4(AsyncMigrateKey,
                void(const std::string& username,
                     const std::string& from_key,
                     const std::string& to_key,
-                    base::Callback<void(int async_id)> callback));
+                    const AsyncMethodCallback& callback));
   MOCK_METHOD2(AsyncRemove, void(const std::string& username,
-                                 base::Callback<void(int async_id)> callback));
+                                 const AsyncMethodCallback& callback));
   MOCK_METHOD1(GetSystemSalt, bool(std::vector<uint8>* salt));
   MOCK_METHOD4(AsyncMount, void(const std::string& username,
                                 const std::string& key,
                                 const bool create_if_missing,
-                                base::Callback<void(int async_id)> callback));
+                                const AsyncMethodCallback& callback));
   MOCK_METHOD1(AsyncMountGuest,
-               void(base::Callback<void(int async_id)> callback));
+               void(const AsyncMethodCallback& callback));
   MOCK_METHOD1(TpmIsReady, bool(bool* ready));
-  MOCK_METHOD1(TpmIsEnabled, void(BoolMethodCallback callback));
+  MOCK_METHOD1(TpmIsEnabled, void(const BoolMethodCallback& callback));
   MOCK_METHOD1(CallTpmIsEnabledAndBlock, bool(bool* enabled));
   MOCK_METHOD1(TpmGetPassword, bool(std::string* password));
   MOCK_METHOD1(TpmIsOwned, bool(bool* owned));
   MOCK_METHOD1(TpmIsBeingOwned, bool(bool* owning));
   MOCK_METHOD0(TpmCanAttemptOwnership, bool());
   MOCK_METHOD0(TpmClearStoredPassword, bool());
-  MOCK_METHOD1(Pkcs11IsTpmTokenReady, void(BoolMethodCallback callback));
+  MOCK_METHOD1(Pkcs11IsTpmTokenReady, void(const BoolMethodCallback& callback));
   MOCK_METHOD1(Pkcs11GetTpmTokenInfo,
-               void(Pkcs11GetTpmTokenInfoCallback callback));
+               void(const Pkcs11GetTpmTokenInfoCallback& callback));
   MOCK_METHOD3(InstallAttributesGet,
                bool(const std::string& name,
                     std::vector<uint8>* value,
diff --git a/chromeos/dbus/mock_image_burner_client.h b/chromeos/dbus/mock_image_burner_client.h
index 8db0a44..6cb31fe 100644
--- a/chromeos/dbus/mock_image_burner_client.h
+++ b/chromeos/dbus/mock_image_burner_client.h
@@ -18,10 +18,11 @@ class MockImageBurnerClient : public ImageBurnerClient {
   MockImageBurnerClient();
   virtual ~MockImageBurnerClient();
 
-  MOCK_METHOD3(BurnImage,
-               void(const std::string&, const std::string&, ErrorCallback));
-  MOCK_METHOD2(SetEventHandlers,
-                void(BurnFinishedHandler, BurnProgressUpdateHandler));
+  MOCK_METHOD3(BurnImage, void(const std::string&,
+                               const std::string&,
+                               const ErrorCallback&));
+  MOCK_METHOD2(SetEventHandlers, void(const BurnFinishedHandler&,
+                                      const BurnProgressUpdateHandler&));
   MOCK_METHOD0(ResetEventHandlers, void());
 
 };
diff --git a/chromeos/dbus/mock_power_manager_client.h b/chromeos/dbus/mock_power_manager_client.h
index e4eb030..81e8fdd 100644
--- a/chromeos/dbus/mock_power_manager_client.h
+++ b/chromeos/dbus/mock_power_manager_client.h
@@ -31,10 +31,11 @@ class MockPowerManagerClient : public PowerManagerClient {
   MOCK_METHOD1(CalculateIdleTime, void(const CalculateIdleTimeCallback&));
   MOCK_METHOD1(RequestIdleNotification, void(int64));
   MOCK_METHOD0(RequestActiveNotification, void(void));
-  MOCK_METHOD4(RequestPowerStateOverrides, void(uint32,
-                                                uint32,
-                                                int,
-                                                PowerStateRequestIdCallback));
+  MOCK_METHOD4(RequestPowerStateOverrides,
+               void(uint32,
+                    uint32,
+                    int,
+                    const PowerStateRequestIdCallback&));
   MOCK_METHOD0(NotifyScreenLockRequested, void(void));
   MOCK_METHOD0(NotifyScreenLockCompleted, void(void));
   MOCK_METHOD0(NotifyScreenUnlockRequested, void(void));
diff --git a/chromeos/dbus/mock_session_manager_client.h b/chromeos/dbus/mock_session_manager_client.h
index 6ed73bb..b8632b54 100644
--- a/chromeos/dbus/mock_session_manager_client.h
+++ b/chromeos/dbus/mock_session_manager_client.h
@@ -25,12 +25,12 @@ class MockSessionManagerClient : public SessionManagerClient {
   MOCK_METHOD2(RestartJob, void(int, const std::string&));
   MOCK_METHOD1(StartSession, void(const std::string&));
   MOCK_METHOD0(StopSession, void(void));
-  MOCK_METHOD1(RetrieveDevicePolicy, void(RetrievePolicyCallback));
-  MOCK_METHOD1(RetrieveUserPolicy, void(RetrievePolicyCallback));
+  MOCK_METHOD1(RetrieveDevicePolicy, void(const RetrievePolicyCallback&));
+  MOCK_METHOD1(RetrieveUserPolicy, void(const RetrievePolicyCallback&));
   MOCK_METHOD2(StoreDevicePolicy, void(const std::string&,
-                                       StorePolicyCallback));
+                                       const StorePolicyCallback&));
   MOCK_METHOD2(StoreUserPolicy, void(const std::string&,
-                                     StorePolicyCallback));
+                                     const StorePolicyCallback&));
 };
 
 }  // namespace chromeos
diff --git a/chromeos/dbus/mock_update_engine_client.h b/chromeos/dbus/mock_update_engine_client.h
index 385a0c5..70cff66 100644
--- a/chromeos/dbus/mock_update_engine_client.h
+++ b/chromeos/dbus/mock_update_engine_client.h
@@ -20,10 +20,10 @@ class MockUpdateEngineClient : public UpdateEngineClient {
   MOCK_METHOD1(AddObserver, void(Observer*));
   MOCK_METHOD1(RemoveObserver, void(Observer*));
   MOCK_METHOD1(HasObserver, bool(Observer*));
-  MOCK_METHOD1(RequestUpdateCheck, void(UpdateCheckCallback));
+  MOCK_METHOD1(RequestUpdateCheck, void(const UpdateCheckCallback&));
   MOCK_METHOD0(RebootAfterUpdate, void());
   MOCK_METHOD1(SetReleaseTrack, void(const std::string&));
-  MOCK_METHOD1(GetReleaseTrack, void(GetReleaseTrackCallback));
+  MOCK_METHOD1(GetReleaseTrack, void(const GetReleaseTrackCallback&));
   MOCK_METHOD0(GetLastStatus, Status());
 };
 
diff --git a/chromeos/dbus/power_manager_client.cc b/chromeos/dbus/power_manager_client.cc
index 43532aa..010b45c 100644
--- a/chromeos/dbus/power_manager_client.cc
+++ b/chromeos/dbus/power_manager_client.cc
@@ -224,7 +224,7 @@ class PowerManagerClientImpl : public PowerManagerClient {
       uint32 request_id,
       uint32 duration,
       int overrides,
-      PowerStateRequestIdCallback callback) OVERRIDE {
+      const PowerStateRequestIdCallback& callback) OVERRIDE {
     dbus::MethodCall method_call(power_manager::kPowerManagerInterface,
                                  power_manager::kStateOverrideRequest);
     dbus::MessageWriter writer(&method_call);
@@ -544,7 +544,7 @@ class PowerManagerClientStubImpl : public PowerManagerClient {
       uint32 request_id,
       uint32 duration,
       int overrides,
-      PowerStateRequestIdCallback callback) OVERRIDE {}
+      const PowerStateRequestIdCallback& callback) OVERRIDE {}
 
   virtual void NotifyScreenLockRequested() OVERRIDE {
     FOR_EACH_OBSERVER(Observer, observers_, LockScreen());
diff --git a/chromeos/dbus/power_manager_client.h b/chromeos/dbus/power_manager_client.h
index 3100de8..eb19947 100644
--- a/chromeos/dbus/power_manager_client.h
+++ b/chromeos/dbus/power_manager_client.h
@@ -162,7 +162,7 @@ class CHROMEOS_EXPORT PowerManagerClient {
       uint32 request_id,
       uint32 duration,
       int overrides,
-      PowerStateRequestIdCallback callback) = 0;
+      const PowerStateRequestIdCallback& callback) = 0;
 
   // Creates the instance.
   static PowerManagerClient* Create(DBusClientImplementationType type,
diff --git a/chromeos/dbus/session_manager_client.cc b/chromeos/dbus/session_manager_client.cc
index b96d2ad..6c40353 100644
--- a/chromeos/dbus/session_manager_client.cc
+++ b/chromeos/dbus/session_manager_client.cc
@@ -134,27 +134,29 @@ class SessionManagerClientImpl : public SessionManagerClient {
   }
 
   // SessionManagerClient override.
-  virtual void RetrieveDevicePolicy(RetrievePolicyCallback callback) OVERRIDE {
+  virtual void RetrieveDevicePolicy(
+      const RetrievePolicyCallback& callback) OVERRIDE {
     CallRetrievePolicy(login_manager::kSessionManagerRetrievePolicy,
                        callback);
   }
 
   // SessionManagerClient override.
-  virtual void RetrieveUserPolicy(RetrievePolicyCallback callback) OVERRIDE {
+  virtual void RetrieveUserPolicy(
+      const RetrievePolicyCallback& callback) OVERRIDE {
     CallRetrievePolicy(login_manager::kSessionManagerRetrieveUserPolicy,
                        callback);
   }
 
   // SessionManagerClient override.
   virtual void StoreDevicePolicy(const std::string& policy_blob,
-                                 StorePolicyCallback callback) OVERRIDE {
+                                 const StorePolicyCallback& callback) OVERRIDE {
     CallStorePolicy(login_manager::kSessionManagerStorePolicy,
                     policy_blob, callback);
   }
 
   // SessionManagerClient override.
   virtual void StoreUserPolicy(const std::string& policy_blob,
-                               StorePolicyCallback callback) OVERRIDE {
+                               const StorePolicyCallback& callback) OVERRIDE {
     CallStorePolicy(login_manager::kSessionManagerStoreUserPolicy,
                     policy_blob, callback);
   }
@@ -162,7 +164,7 @@ class SessionManagerClientImpl : public SessionManagerClient {
  private:
   // Helper for Retrieve{User,Device}Policy.
   virtual void CallRetrievePolicy(const std::string& method_name,
-                                  RetrievePolicyCallback callback) {
+                                  const RetrievePolicyCallback& callback) {
     dbus::MethodCall method_call(login_manager::kSessionManagerInterface,
                                  method_name);
     session_manager_proxy_->CallMethod(
@@ -177,7 +179,7 @@ class SessionManagerClientImpl : public SessionManagerClient {
   // Helper for Store{User,Device}Policy.
   virtual void CallStorePolicy(const std::string& method_name,
                                const std::string& policy_blob,
-                               StorePolicyCallback callback) {
+                               const StorePolicyCallback& callback) {
     dbus::MethodCall method_call(login_manager::kSessionManagerInterface,
                                  method_name);
     dbus::MessageWriter writer(&method_call);
@@ -238,7 +240,7 @@ class SessionManagerClientImpl : public SessionManagerClient {
   // Called when kSessionManagerRetrievePolicy or
   // kSessionManagerRetrieveUserPolicy  method is complete.
   void OnRetrievePolicy(const std::string& method_name,
-                        RetrievePolicyCallback callback,
+                        const RetrievePolicyCallback& callback,
                         dbus::Response* response) {
     if (!response) {
       LOG(ERROR) << "Failed to call " << method_name;
@@ -261,7 +263,7 @@ class SessionManagerClientImpl : public SessionManagerClient {
   // Called when kSessionManagerStorePolicy or kSessionManagerStoreUserPolicy
   // method is complete.
   void OnStorePolicy(const std::string& method_name,
-                     StorePolicyCallback callback,
+                     const StorePolicyCallback& callback,
                      dbus::Response* response) {
     bool success = false;
     if (!response) {
@@ -324,18 +326,20 @@ class SessionManagerClientStubImpl : public SessionManagerClient {
   virtual void RestartEntd() OVERRIDE {}
   virtual void StartSession(const std::string& user_email) OVERRIDE {}
   virtual void StopSession() OVERRIDE {}
-  virtual void RetrieveDevicePolicy(RetrievePolicyCallback callback) OVERRIDE {
+  virtual void RetrieveDevicePolicy(
+      const RetrievePolicyCallback& callback) OVERRIDE {
     callback.Run("");
   }
-  virtual void RetrieveUserPolicy(RetrievePolicyCallback callback) OVERRIDE {
+  virtual void RetrieveUserPolicy(
+      const RetrievePolicyCallback& callback) OVERRIDE {
     callback.Run("");
   }
   virtual void StoreDevicePolicy(const std::string& policy_blob,
-                                 StorePolicyCallback callback) OVERRIDE {
+                                 const StorePolicyCallback& callback) OVERRIDE {
     callback.Run(true);
   }
   virtual void StoreUserPolicy(const std::string& policy_blob,
-                               StorePolicyCallback callback) OVERRIDE {
+                               const StorePolicyCallback& callback) OVERRIDE {
     callback.Run(true);
   }
 };
diff --git a/chromeos/dbus/session_manager_client.h b/chromeos/dbus/session_manager_client.h
index c6903f9..82382e3 100644
--- a/chromeos/dbus/session_manager_client.h
+++ b/chromeos/dbus/session_manager_client.h
@@ -60,12 +60,12 @@ class CHROMEOS_EXPORT SessionManagerClient {
 
   // Fetches the device policy blob stored by the session manager.  Upon
   // completion of the retrieve attempt, we will call the provided callback.
-  virtual void RetrieveDevicePolicy(RetrievePolicyCallback callback) = 0;
+  virtual void RetrieveDevicePolicy(const RetrievePolicyCallback& callback) = 0;
 
   // Fetches the user policy blob stored by the session manager for the
   // currently signed-in user.  Upon completion of the retrieve attempt, we will
   // call the provided callback.
-  virtual void RetrieveUserPolicy(RetrievePolicyCallback callback) = 0;
+  virtual void RetrieveUserPolicy(const RetrievePolicyCallback& callback) = 0;
 
   // Used for StoreDevicePolicy and StoreUserPolicy. Takes a boolean indicating
   // whether the operation was successful or not.
@@ -74,13 +74,13 @@ class CHROMEOS_EXPORT SessionManagerClient {
   // Attempts to asynchronously store |policy_blob| as device policy.  Upon
   // completion of the store attempt, we will call callback.
   virtual void StoreDevicePolicy(const std::string& policy_blob,
-                                 StorePolicyCallback callback) = 0;
+                                 const StorePolicyCallback& callback) = 0;
 
   // Attempts to asynchronously store |policy_blob| as user policy for the
   // currently signed-in user.  Upon completion of the store attempt, we will
   // call callback.
   virtual void StoreUserPolicy(const std::string& policy_blob,
-                               StorePolicyCallback callback) = 0;
+                               const StorePolicyCallback& callback) = 0;
 
   // Creates the instance.
   static SessionManagerClient* Create(DBusClientImplementationType type,
diff --git a/chromeos/dbus/update_engine_client.cc b/chromeos/dbus/update_engine_client.cc
index 55f5c68..5e3e21a 100644
--- a/chromeos/dbus/update_engine_client.cc
+++ b/chromeos/dbus/update_engine_client.cc
@@ -87,7 +87,8 @@ class UpdateEngineClientImpl : public UpdateEngineClient {
   }
 
   // UpdateEngineClient override.
-  virtual void RequestUpdateCheck(UpdateCheckCallback callback) OVERRIDE {
+  virtual void RequestUpdateCheck(
+      const UpdateCheckCallback& callback) OVERRIDE {
     dbus::MethodCall method_call(
         update_engine::kUpdateEngineInterface,
         update_engine::kAttemptUpdate);
@@ -135,7 +136,8 @@ class UpdateEngineClientImpl : public UpdateEngineClient {
   }
 
   // UpdateEngineClient override.
-  virtual void GetReleaseTrack(GetReleaseTrackCallback callback) OVERRIDE {
+  virtual void GetReleaseTrack(
+      const GetReleaseTrackCallback& callback) OVERRIDE {
     dbus::MethodCall method_call(
         update_engine::kUpdateEngineInterface,
         update_engine::kGetTrack);
@@ -156,7 +158,7 @@ class UpdateEngineClientImpl : public UpdateEngineClient {
 
  private:
   // Called when a response for RequestUpdateCheck() is received.
-  void OnRequestUpdateCheck(UpdateCheckCallback callback,
+  void OnRequestUpdateCheck(const UpdateCheckCallback& callback,
                             dbus::Response* response) {
     if (!response) {
       LOG(ERROR) << "Failed to request update check";
@@ -183,7 +185,7 @@ class UpdateEngineClientImpl : public UpdateEngineClient {
   }
 
   // Called when a response for GetReleaseTrack() is received.
-  void OnGetReleaseTrack(GetReleaseTrackCallback callback,
+  void OnGetReleaseTrack(const GetReleaseTrackCallback& callback,
                          dbus::Response* response) {
     if (!response) {
       LOG(ERROR) << "Failed to request getting release track";
@@ -254,12 +256,14 @@ class UpdateEngineClientStubImpl : public UpdateEngineClient {
   virtual void RemoveObserver(Observer* observer) OVERRIDE {}
   virtual bool HasObserver(Observer* observer) OVERRIDE { return false; }
 
-  virtual void RequestUpdateCheck(UpdateCheckCallback callback) OVERRIDE {
+  virtual void RequestUpdateCheck(
+      const UpdateCheckCallback& callback) OVERRIDE {
     callback.Run(UPDATE_RESULT_NOTIMPLEMENTED);
   }
   virtual void RebootAfterUpdate() OVERRIDE {}
   virtual void SetReleaseTrack(const std::string& track) OVERRIDE {}
-  virtual void GetReleaseTrack(GetReleaseTrackCallback callback) OVERRIDE {
+  virtual void GetReleaseTrack(
+      const GetReleaseTrackCallback& callback) OVERRIDE {
     callback.Run("beta-channel");
   }
   virtual Status GetLastStatus() OVERRIDE { return Status(); }
diff --git a/chromeos/dbus/update_engine_client.h b/chromeos/dbus/update_engine_client.h
index 7d1692d..914094e 100644
--- a/chromeos/dbus/update_engine_client.h
+++ b/chromeos/dbus/update_engine_client.h
@@ -82,7 +82,7 @@ class CHROMEOS_EXPORT UpdateEngineClient {
   typedef base::Callback<void(UpdateCheckResult)> UpdateCheckCallback;
 
   // Requests an update check and calls |callback| when completed.
-  virtual void RequestUpdateCheck(UpdateCheckCallback callback) = 0;
+  virtual void RequestUpdateCheck(const UpdateCheckCallback& callback) = 0;
 
   // Reboots if update has been performed.
   virtual void RebootAfterUpdate() = 0;
@@ -98,7 +98,7 @@ class CHROMEOS_EXPORT UpdateEngineClient {
   // Requests to get the release track and calls |callback| with the
   // release track (channel). On error, calls |callback| with an empty
   // string.
-  virtual void GetReleaseTrack(GetReleaseTrackCallback callback) = 0;
+  virtual void GetReleaseTrack(const GetReleaseTrackCallback& callback) = 0;
 
   // Returns the last status the object received from the update engine.
   //