Revert 221966 "Revert 221944 "Add UMA to report Preferences File..."

Re-apply and disable for Chrome Frame.

> Revert 221944 "Add UMA to report Preferences File Corruption"
> 
> Breaks chrome_frame_tests on Win Aura.
> http://test-results.appspot.com/dashboards/flakiness_dashboard.html#group=%40ToT%20Chromium&testType=chrome_frame_tests&builder=Win%20Aura%20Tests%20(1)
> 
> > Add UMA to report Preferences File Corruption
> > Adds PrefMetricsService methods to track changes to user preferences
> > that happen outside of Chrome and report discrepancies via UMA.
> > 
> > BUG=266569
> > 
> > Review URL: https://chromiumcodereview.appspot.com/22676002
> 
> TBR=bbudge@chromium.org
> 
> Review URL: https://codereview.chromium.org/23597020

TBR=bbudge@chromium.org

Review URL: https://codereview.chromium.org/23788010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@221972 0039d316-1c4b-4281-b951-d872f2087c98

9 files changed, 612 insertions, 18 deletions

diff --git a/chrome/browser/browser_resources.grd b/chrome/browser/browser_resources.grd
index 5c441ba..448b7c4 100644
--- a/chrome/browser/browser_resources.grd
+++ b/chrome/browser/browser_resources.grd
@@ -378,6 +378,9 @@
         <include name="IDR_NETWORK_JS" file="resources\chromeos\network.js" type="BINDATA" />
         <include name="IDR_NETWORK_CSS" file="resources\chromeos\network.css" type="BINDATA" />
       </if>
+      <if expr="pp_ifdef('_google_chrome')">
+        <include name="IDR_PREF_HASH_SEED_BIN" file="resources\settings_internal\pref_hash_seed.bin" type="BINDATA" />
+      </if>
     </includes>
   </release>
 </grit>
diff --git a/chrome/browser/prefs/browser_prefs.cc b/chrome/browser/prefs/browser_prefs.cc
index 5d4c679..990249f 100644
--- a/chrome/browser/prefs/browser_prefs.cc
+++ b/chrome/browser/prefs/browser_prefs.cc
@@ -52,6 +52,7 @@
 #include "chrome/browser/pepper_flash_settings_manager.h"
 #include "chrome/browser/plugins/plugin_finder.h"
 #include "chrome/browser/prefs/incognito_mode_prefs.h"
+#include "chrome/browser/prefs/pref_metrics_service.h"
 #include "chrome/browser/prefs/pref_service_syncable.h"
 #include "chrome/browser/prefs/session_startup_pref.h"
 #include "chrome/browser/printing/cloud_print/cloud_print_url.h"
@@ -217,6 +218,7 @@ void RegisterLocalState(PrefRegistrySimple* registry) {
   MetricsLog::RegisterPrefs(registry);
   MetricsService::RegisterPrefs(registry);
   metrics::CachingPermutedEntropyProvider::RegisterPrefs(registry);
+  PrefMetricsService::RegisterPrefs(registry);
   PrefProxyConfigTrackerImpl::RegisterPrefs(registry);
   ProfileInfoCache::RegisterPrefs(registry);
   profiles::RegisterPrefs(registry);
diff --git a/chrome/browser/prefs/pref_metrics_service.cc b/chrome/browser/prefs/pref_metrics_service.cc
index 3814032..9081706 100644
--- a/chrome/browser/prefs/pref_metrics_service.cc
+++ b/chrome/browser/prefs/pref_metrics_service.cc
@@ -5,48 +5,119 @@
 #include "chrome/browser/prefs/pref_metrics_service.h"
 
 #include "base/bind.h"
+#include "base/command_line.h"
+#include "base/json/json_string_value_serializer.h"
 #include "base/metrics/histogram.h"
+#include "base/prefs/pref_registry_simple.h"
 #include "base/prefs/pref_service.h"
+#include "base/strings/string_number_conversions.h"
+#include "chrome/browser/browser_process.h"
+#include "chrome/browser/browser_shutdown.h"
+// Accessing the Device ID API here is a layering violation.
+// TODO(bbudge) Move the API so it's usable here.
+// http://crbug.com/276485
+#include "chrome/browser/extensions/api/music_manager_private/device_id.h"
+#include "chrome/browser/extensions/extension_prefs.h"
 #include "chrome/browser/prefs/pref_service_syncable.h"
+#include "chrome/browser/prefs/scoped_user_pref_update.h"
 #include "chrome/browser/prefs/session_startup_pref.h"
 #include "chrome/browser/prefs/synced_pref_change_registrar.h"
 #include "chrome/browser/profiles/incognito_helpers.h"
 #include "chrome/browser/profiles/profile.h"
 #include "chrome/browser/search_engines/template_url_prepopulate_data.h"
+#include "chrome/common/chrome_switches.h"
 #include "chrome/common/pref_names.h"
 #include "components/browser_context_keyed_service/browser_context_dependency_manager.h"
+#include "crypto/hmac.h"
+#include "grit/browser_resources.h"
 #include "net/base/registry_controlled_domains/registry_controlled_domain.h"
+#include "ui/base/resource/resource_bundle.h"
 
 namespace {
 
-// Converts a host name into a domain name for easier matching.
-std::string GetDomainFromHost(const std::string& host) {
-  return net::registry_controlled_domains::GetDomainAndRegistry(
-      host,
-      net::registry_controlled_domains::EXCLUDE_PRIVATE_REGISTRIES);
-}
-
 const int kSessionStartupPrefValueMax = SessionStartupPref::kPrefValueMax;
 
+// These preferences must be kept in sync with the TrackedPreference enum in
+// tools/metrics/histograms/histograms.xml. To add a new preference, append it
+// to the array and add a corresponding value to the histogram enum.
+const char* kTrackedPrefs[] = {
+  prefs::kShowHomeButton,
+  prefs::kHomePageIsNewTabPage,
+  prefs::kHomePage,
+  prefs::kRestoreOnStartup,
+  prefs::kURLsToRestoreOnStartup,
+  prefs::kExtensionsPref,
+  prefs::kGoogleServicesLastUsername,
+  prefs::kSearchProviderOverrides,
+  prefs::kDefaultSearchProviderSearchURL,
+  prefs::kDefaultSearchProviderKeyword,
+  prefs::kDefaultSearchProviderName,
+#if !defined(OS_ANDROID)
+  prefs::kPinnedTabs,
+#endif
+};
+
+static const size_t kSHA256DigestSize = 32;
+
 }  // namespace
 
 PrefMetricsService::PrefMetricsService(Profile* profile)
-    : profile_(profile) {
+    : profile_(profile),
+      prefs_(profile_->GetPrefs()),
+      local_state_(g_browser_process->local_state()),
+      profile_name_(profile_->GetPath().AsUTF8Unsafe()),
+      tracked_pref_paths_(kTrackedPrefs),
+      tracked_pref_path_count_(arraysize(kTrackedPrefs)),
+      checked_tracked_prefs_(false),
+      weak_factory_(this) {
+  pref_hash_seed_ = ResourceBundle::GetSharedInstance().GetRawDataResource(
+      IDR_PREF_HASH_SEED_BIN).as_string();
+
   RecordLaunchPrefs();
 
   PrefServiceSyncable* prefs = PrefServiceSyncable::FromProfile(profile_);
   synced_pref_change_registrar_.reset(new SyncedPrefChangeRegistrar(prefs));
 
   RegisterSyncedPrefObservers();
+
+  // The following code might cause callbacks into this instance before we exit
+  // the constructor. This instance should be initialized at this point.
+
+  // Android has no GetDeviceId.
+#if !defined(OS_ANDROID)
+  // We need the machine id to compute pref value hashes. Fetch that, and then
+  // call CheckTrackedPreferences in the callback.
+  extensions::api::DeviceId::GetDeviceId(
+      "PrefMetricsService",  // non-empty string to obfuscate the device id.
+      Bind(&PrefMetricsService::GetDeviceIdCallback,
+           weak_factory_.GetWeakPtr()));
+#endif  // !defined(OS_ANDROID)
+}
+
+// For unit testing only.
+PrefMetricsService::PrefMetricsService(Profile* profile,
+                                       PrefService* local_state,
+                                       const std::string& device_id,
+                                       const char** tracked_pref_paths,
+                                       int tracked_pref_path_count)
+    : profile_(profile),
+      prefs_(profile->GetPrefs()),
+      local_state_(local_state),
+      pref_hash_seed_(kSHA256DigestSize, 0),
+      device_id_(device_id),
+      tracked_pref_paths_(tracked_pref_paths),
+      tracked_pref_path_count_(tracked_pref_path_count),
+      checked_tracked_prefs_(false),
+      weak_factory_(this) {
+  CheckTrackedPreferences();
 }
 
 PrefMetricsService::~PrefMetricsService() {
 }
 
 void PrefMetricsService::RecordLaunchPrefs() {
-  PrefService* prefs = profile_->GetPrefs();
-  bool show_home_button = prefs->GetBoolean(prefs::kShowHomeButton);
-  bool home_page_is_ntp = prefs->GetBoolean(prefs::kHomePageIsNewTabPage);
+  bool show_home_button = prefs_->GetBoolean(prefs::kShowHomeButton);
+  bool home_page_is_ntp = prefs_->GetBoolean(prefs::kHomePageIsNewTabPage);
   UMA_HISTOGRAM_BOOLEAN("Settings.ShowHomeButton", show_home_button);
   if (show_home_button) {
     UMA_HISTOGRAM_BOOLEAN("Settings.GivenShowHomeButton_HomePageIsNewTabPage",
@@ -60,7 +131,7 @@ void PrefMetricsService::RecordLaunchPrefs() {
   // (pages that share a TLD+1 with a known engine but aren't actually search
   // pages, e.g. plus.google.com).
   if (!home_page_is_ntp) {
-    GURL homepage_url(prefs->GetString(prefs::kHomePage));
+    GURL homepage_url(prefs_->GetString(prefs::kHomePage));
     if (homepage_url.is_valid()) {
       UMA_HISTOGRAM_ENUMERATION(
           "Settings.HomePageEngineType",
@@ -69,11 +140,11 @@ void PrefMetricsService::RecordLaunchPrefs() {
     }
   }
 
-  int restore_on_startup = prefs->GetInteger(prefs::kRestoreOnStartup);
+  int restore_on_startup = prefs_->GetInteger(prefs::kRestoreOnStartup);
   UMA_HISTOGRAM_ENUMERATION("Settings.StartupPageLoadSettings",
                             restore_on_startup, kSessionStartupPrefValueMax);
   if (restore_on_startup == SessionStartupPref::kPrefValueURLs) {
-    const ListValue* url_list = prefs->GetList(prefs::kURLsToRestoreOnStartup);
+    const ListValue* url_list = prefs_->GetList(prefs::kURLsToRestoreOnStartup);
     UMA_HISTOGRAM_CUSTOM_COUNTS("Settings.StartupPageLoadURLs",
                                 url_list->GetSize(), 1, 50, 20);
     // Similarly, check startup pages for known search engine TLD+1s.
@@ -92,6 +163,13 @@ void PrefMetricsService::RecordLaunchPrefs() {
   }
 }
 
+// static
+void PrefMetricsService::RegisterPrefs(PrefRegistrySimple* registry) {
+  // Register the top level dictionary to map profile names to dictionaries of
+  // tracked preferences.
+  registry->RegisterDictionaryPref(prefs::kProfilePreferenceHashes);
+}
+
 void PrefMetricsService::RegisterSyncedPrefObservers() {
   LogHistogramValueCallback booleanHandler = base::Bind(
       &PrefMetricsService::LogBooleanPrefChange, base::Unretained(this));
@@ -169,6 +247,142 @@ void PrefMetricsService::LogListPrefChange(
   }
 }
 
+void PrefMetricsService::GetDeviceIdCallback(const std::string& device_id) {
+  device_id_ = device_id;
+  // On Aura, this seems to be called twice.
+  if (!checked_tracked_prefs_)
+    CheckTrackedPreferences();
+}
+
+// To detect changes to Preferences that happen outside of Chrome, we hash
+// selected pref values and save them in local state. CheckTrackedPreferences
+// compares the saved values to the values observed in the profile's prefs. A
+// dictionary of dictionaries in local state holds the hashed values, grouped by
+// profile. To make the system more resistant to spoofing, pref values are
+// hashed with the pref path and the device id.
+void PrefMetricsService::CheckTrackedPreferences() {
+  DCHECK(!checked_tracked_prefs_);
+
+  const base::DictionaryValue* pref_hash_dicts =
+      local_state_->GetDictionary(prefs::kProfilePreferenceHashes);
+  // Get the hashed prefs dictionary if it exists. If it doesn't, it will be
+  // created if we set preference values below.
+  const base::DictionaryValue* hashed_prefs = NULL;
+  pref_hash_dicts->GetDictionary(profile_name_, &hashed_prefs);
+  for (int i = 0; i < tracked_pref_path_count_; ++i) {
+    // Skip prefs that haven't been registered.
+    if (!prefs_->FindPreference(tracked_pref_paths_[i]))
+      continue;
+
+    bool changed = false;
+    const base::Value* value = prefs_->GetUserPrefValue(tracked_pref_paths_[i]);
+    if (value) {
+      std::string value_hash =
+          GetHashedPrefValue(tracked_pref_paths_[i], value);
+      std::string last_hash;
+      if (hashed_prefs &&
+          hashed_prefs->GetString(tracked_pref_paths_[i], &last_hash)) {
+        if (value_hash != last_hash) {
+          changed = true;
+          // Record that the preference changed from its last value.
+          UMA_HISTOGRAM_ENUMERATION("Settings.TrackedPreferenceChanged",
+              i, tracked_pref_path_count_);
+          UpdateTrackedPreference(tracked_pref_paths_[i]);
+        }
+      } else {
+        changed = true;
+        // Record that we haven't tracked this preference yet, or the hash in
+        // local state was removed.
+        UMA_HISTOGRAM_ENUMERATION("Settings.TrackedPreferenceInitialized",
+            i, tracked_pref_path_count_);
+        UpdateTrackedPreference(tracked_pref_paths_[i]);
+      }
+    } else {
+      // There is no preference set. Remove any hashed value from local state
+      // and if one was present, record that a pref was cleared.
+      if (RemoveTrackedPreference(tracked_pref_paths_[i])) {
+        changed = true;
+        UMA_HISTOGRAM_ENUMERATION("Settings.TrackedPreferenceCleared",
+            i, tracked_pref_path_count_);
+      }
+    }
+    if (!changed) {
+      UMA_HISTOGRAM_ENUMERATION("Settings.TrackedPreferenceUnchanged",
+          i, tracked_pref_path_count_);
+    }
+  }
+
+  checked_tracked_prefs_ = true;
+
+  // Now that we've checked the incoming preferences, register for change
+  // notifications, unless this is test code.
+  // TODO(bbudge) Fix failing browser_tests and so we can remove this test.
+  // Several tests fail when they shutdown before they can write local state.
+  if (!CommandLine::ForCurrentProcess()->HasSwitch(switches::kTestType) &&
+      !CommandLine::ForCurrentProcess()->HasSwitch(switches::kChromeFrame)) {
+    InitializePrefObservers();
+  }
+}
+
+void PrefMetricsService::UpdateTrackedPreference(const char* path) {
+  const base::Value* value = prefs_->GetUserPrefValue(path);
+  if (value) {
+    DictionaryPrefUpdate update(local_state_, prefs::kProfilePreferenceHashes);
+    update->SetString(GetHashedPrefPath(path),
+                      GetHashedPrefValue(path, value));
+  } else {
+    RemoveTrackedPreference(path);
+  }
+}
+
+bool PrefMetricsService::RemoveTrackedPreference(const char* path) {
+  DictionaryPrefUpdate update(local_state_, prefs::kProfilePreferenceHashes);
+  return update->Remove(GetHashedPrefPath(path), NULL);
+}
+
+std::string PrefMetricsService::GetHashedPrefPath(const char* path) {
+  std::string hash_pref_path(profile_name_);
+  hash_pref_path.append(".");
+  hash_pref_path.append(path);
+  return hash_pref_path;
+}
+
+std::string PrefMetricsService::GetHashedPrefValue(
+    const char* path,
+    const base::Value* value) {
+  DCHECK(value);
+
+  std::string string_to_hash(device_id_);
+  string_to_hash.append(path);
+  JSONStringValueSerializer serializer(&string_to_hash);
+  serializer.Serialize(*value);
+
+  crypto::HMAC hmac(crypto::HMAC::SHA256);
+  unsigned char digest[kSHA256DigestSize];
+  if (!hmac.Init(pref_hash_seed_) ||
+      !hmac.Sign(string_to_hash, digest, kSHA256DigestSize)) {
+    NOTREACHED();
+    return std::string();
+  }
+
+  return base::HexEncode(digest, kSHA256DigestSize);
+}
+
+void PrefMetricsService::InitializePrefObservers() {
+  pref_registrar_.Init(prefs_);
+  for (int i = 0; i < tracked_pref_path_count_; ++i) {
+    // Skip prefs that haven't been registered.
+    if (!prefs_->FindPreference(tracked_pref_paths_[i]))
+      continue;
+
+    pref_registrar_.Add(
+        tracked_pref_paths_[i],
+        base::Bind(&PrefMetricsService::UpdateTrackedPreference,
+                   weak_factory_.GetWeakPtr(),
+                   tracked_pref_paths_[i]));
+  }
+}
+
 // static
 PrefMetricsService::Factory* PrefMetricsService::Factory::GetInstance() {
   return Singleton<PrefMetricsService::Factory>::get();
diff --git a/chrome/browser/prefs/pref_metrics_service.h b/chrome/browser/prefs/pref_metrics_service.h
index fc48935..b2657c1 100644
--- a/chrome/browser/prefs/pref_metrics_service.h
+++ b/chrome/browser/prefs/pref_metrics_service.h
@@ -10,11 +10,15 @@
 
 #include "base/memory/scoped_ptr.h"
 #include "base/memory/singleton.h"
+#include "base/memory/weak_ptr.h"
+#include "base/prefs/pref_change_registrar.h"
 #include "chrome/browser/prefs/synced_pref_change_registrar.h"
 #include "chrome/browser/profiles/profile.h"
 #include "components/browser_context_keyed_service/browser_context_keyed_service.h"
 #include "components/browser_context_keyed_service/browser_context_keyed_service_factory.h"
 
+class PrefRegistrySimple;
+
 // PrefMetricsService is responsible for recording prefs-related UMA stats.
 class PrefMetricsService : public BrowserContextKeyedService {
  public:
@@ -40,16 +44,23 @@ class PrefMetricsService : public BrowserContextKeyedService {
         content::BrowserContext* context) const OVERRIDE;
   };
 
+  // Registers preferences in local state.
+  static void RegisterPrefs(PrefRegistrySimple* registry);
+
  private:
-  // Use a map to convert domains to their histogram identifiers. Ids are
-  // defined in tools/metrics/histograms/histograms.xml and (usually) also in
-  // chrome/browser/search_engines/prepopulated_engines.json.
-  typedef std::map<std::string, int> DomainIdMap;
+  friend class PrefMetricsServiceTest;
 
   // Function to log a Value to a histogram
   typedef base::Callback<void(const std::string&, const Value*)>
       LogHistogramValueCallback;
 
+  // For unit testing only.
+  PrefMetricsService(Profile* profile,
+                     PrefService* local_settings,
+                     const std::string& device_id,
+                     const char** tracked_pref_paths,
+                     int tracked_pref_path_count);
+
   // Record prefs state on browser context creation.
   void RecordLaunchPrefs();
 
@@ -82,8 +93,45 @@ class PrefMetricsService : public BrowserContextKeyedService {
                          const std::string& histogram_name,
                          const Value* value);
 
+  // Callback to receive a unique device_id.
+  void GetDeviceIdCallback(const std::string& device_id);
+
+  // Checks the tracked preferences against their last known values and reports
+  // any discrepancies. This must be called after |device_id| has been set.
+  void CheckTrackedPreferences();
+
+  // Updates the hash of the tracked preference in local state. This must be
+  // called after |device_id| has been set.
+  void UpdateTrackedPreference(const char* path);
+
+  // Removes the tracked preference from local state. Returns 'true' iff. the
+  // value was present.
+  bool RemoveTrackedPreference(const char* path);
+
+  // Gets the path to the preference value hash in local state.
+  std::string GetHashedPrefPath(const char* path);
+
+  // Computes an MD5 hash for the given preference value.
+  std::string GetHashedPrefValue(const char* path, const base::Value* value);
+
+  void InitializePrefObservers();
+
   Profile* profile_;
+  PrefService* prefs_;
+  PrefService* local_state_;
+  std::string profile_name_;
+  std::string pref_hash_seed_;
+  std::string device_id_;
+  const char** tracked_pref_paths_;
+  const int tracked_pref_path_count_;
+  bool checked_tracked_prefs_;
+
+  PrefChangeRegistrar pref_registrar_;
   scoped_ptr<SyncedPrefChangeRegistrar> synced_pref_change_registrar_;
+
+  base::WeakPtrFactory<PrefMetricsService> weak_factory_;
+
+  DISALLOW_COPY_AND_ASSIGN(PrefMetricsService);
 };
 
 #endif  // CHROME_BROWSER_PREFS_PREF_METRICS_SERVICE_H_
diff --git a/chrome/browser/prefs/pref_metrics_service_unittest.cc b/chrome/browser/prefs/pref_metrics_service_unittest.cc
new file mode 100644
index 0000000..9e05c49
--- /dev/null
+++ b/chrome/browser/prefs/pref_metrics_service_unittest.cc
@@ -0,0 +1,274 @@
+// Copyright 2013 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "base/memory/scoped_ptr.h"
+#include "base/metrics/histogram.h"
+#include "base/metrics/statistics_recorder.h"
+#include "base/prefs/testing_pref_service.h"
+#include "base/values.h"
+#include "chrome/browser/prefs/pref_metrics_service.h"
+#include "chrome/browser/prefs/scoped_user_pref_update.h"
+#include "chrome/test/base/testing_pref_service_syncable.h"
+#include "chrome/test/base/testing_profile.h"
+#include "components/user_prefs/pref_registry_syncable.h"
+#include "testing/gtest/include/gtest/gtest.h"
+
+namespace {
+
+// TestingProfile may register some real preferences; to avoid interference,
+// define fake preferences for testing.
+const char* kTrackedPrefs[] = {
+  "pref_metrics_service_test.pref1",
+  "pref_metrics_service_test.pref2",
+};
+
+const int kTrackedPrefCount = arraysize(kTrackedPrefs);
+
+}  // namespace
+
+class PrefMetricsServiceTest : public testing::Test {
+ protected:
+  virtual void SetUp() {
+    pref1_changed_ = 0;
+    pref2_changed_ = 0;
+    pref1_cleared_ = 0;
+    pref2_cleared_ = 0;
+    pref1_initialized_ = 0;
+    pref2_initialized_ = 0;
+    pref1_unchanged_ = 0;
+    pref2_unchanged_ = 0;
+
+    base::StatisticsRecorder::Initialize();
+
+    prefs_ = profile_.GetTestingPrefService();
+
+    // Register our test-only tracked prefs as string values.
+    for (int i = 0; i < kTrackedPrefCount; ++i) {
+      prefs_->registry()->RegisterStringPref(
+          kTrackedPrefs[i],
+          "test_default_value",
+          user_prefs::PrefRegistrySyncable::SYNCABLE_PREF);
+    }
+
+    // Initialize pref in local state that holds hashed values.
+    PrefMetricsService::RegisterPrefs(local_state_.registry());
+
+    // Update global counts in case another test left stray samples.
+    UpdateHistogramSamples();
+  }
+
+  scoped_ptr<PrefMetricsService> CreatePrefMetricsService() {
+    return scoped_ptr<PrefMetricsService>(
+        new PrefMetricsService(&profile_,
+                               &local_state_,
+                               "test_device_id",
+                               kTrackedPrefs,
+                               kTrackedPrefCount));
+  }
+
+  std::string GetHashedPrefValue(PrefMetricsService* service,
+                                 const char* path, base::Value* value) {
+    return service->GetHashedPrefValue(path, value);
+  }
+
+  void GetSamples(const char* histogram_name, int* bucket1, int* bucket2) {
+    base::HistogramBase* histogram =
+        base::StatisticsRecorder::FindHistogram(histogram_name);
+    if (!histogram) {
+      *bucket1 = 0;
+      *bucket2 = 0;
+    } else {
+      scoped_ptr<base::HistogramSamples> samples(histogram->SnapshotSamples());
+      *bucket1 = samples->GetCount(0);
+      *bucket2 = samples->GetCount(1);
+    }
+  }
+
+  void UpdateHistogramSamples() {
+    int changed1, changed2;
+    GetSamples("Settings.TrackedPreferenceChanged", &changed1, &changed2);
+    pref1_changed_ = changed1 - pref1_changed_total;
+    pref2_changed_ = changed2 - pref2_changed_total;
+    pref1_changed_total = changed1;
+    pref2_changed_total = changed2;
+
+    int cleared1, cleared2;
+    GetSamples("Settings.TrackedPreferenceCleared", &cleared1, &cleared2);
+    pref1_cleared_ = cleared1 - pref1_cleared_total;
+    pref2_cleared_ = cleared2 - pref2_cleared_total;
+    pref1_cleared_total = cleared1;
+    pref2_cleared_total = cleared2;
+
+    int inited1, inited2;
+    GetSamples("Settings.TrackedPreferenceInitialized", &inited1, &inited2);
+    pref1_initialized_ = inited1 - pref1_initialized_total;
+    pref2_initialized_ = inited2 - pref2_initialized_total;
+    pref1_initialized_total = inited1;
+    pref2_initialized_total = inited2;
+
+    int unchanged1, unchanged2;
+    GetSamples("Settings.TrackedPreferenceUnchanged", &unchanged1, &unchanged2);
+    pref1_unchanged_ = unchanged1 - pref1_unchanged_total;
+    pref2_unchanged_ = unchanged2 - pref2_unchanged_total;
+    pref1_unchanged_total = unchanged1;
+    pref2_unchanged_total = unchanged2;
+  }
+
+  TestingProfile profile_;
+  TestingPrefServiceSyncable* prefs_;
+  TestingPrefServiceSimple local_state_;
+
+  // Since histogram samples are recorded by a global StatisticsRecorder, we
+  // need to maintain total counts so we can compute deltas for individual
+  // tests.
+  static int pref1_changed_total;
+  static int pref2_changed_total;
+  static int pref1_cleared_total;
+  static int pref2_cleared_total;
+  static int pref1_initialized_total;
+  static int pref2_initialized_total;
+  static int pref1_unchanged_total;
+  static int pref2_unchanged_total;
+
+  // Counts of samples recorded since UpdateHistogramSamples was last called.
+  int pref1_changed_;
+  int pref2_changed_;
+  int pref1_cleared_;
+  int pref2_cleared_;
+  int pref1_initialized_;
+  int pref2_initialized_;
+  int pref1_unchanged_;
+  int pref2_unchanged_;
+};
+
+int PrefMetricsServiceTest::pref1_changed_total;
+int PrefMetricsServiceTest::pref2_changed_total;
+int PrefMetricsServiceTest::pref1_cleared_total;
+int PrefMetricsServiceTest::pref2_cleared_total;
+int PrefMetricsServiceTest::pref1_initialized_total;
+int PrefMetricsServiceTest::pref2_initialized_total;
+int PrefMetricsServiceTest::pref1_unchanged_total;
+int PrefMetricsServiceTest::pref2_unchanged_total;
+
+TEST_F(PrefMetricsServiceTest, StartupNoUserPref) {
+  // Local state is empty and no user prefs are set. We should record that we
+  // checked preferences once but since there are no user preference values, no
+  // other histogram data should be collected.
+  scoped_ptr<PrefMetricsService> test = CreatePrefMetricsService();
+  UpdateHistogramSamples();
+  EXPECT_EQ(0, pref1_changed_);
+  EXPECT_EQ(0, pref2_changed_);
+  EXPECT_EQ(0, pref1_cleared_);
+  EXPECT_EQ(0, pref2_cleared_);
+  EXPECT_EQ(0, pref1_initialized_);
+  EXPECT_EQ(0, pref2_initialized_);
+  EXPECT_EQ(1, pref1_unchanged_);
+  EXPECT_EQ(1, pref2_unchanged_);
+}
+
+TEST_F(PrefMetricsServiceTest, StartupUserPref) {
+  // Local state is empty. Set a value for one tracked pref. We should record
+  // that we checked preferences once and initialized a hash for the pref.
+  prefs_->SetString(kTrackedPrefs[0], "foo");
+  {
+    scoped_ptr<PrefMetricsService> test = CreatePrefMetricsService();
+    UpdateHistogramSamples();
+    EXPECT_EQ(0, pref1_changed_);
+    EXPECT_EQ(0, pref2_changed_);
+    EXPECT_EQ(0, pref1_cleared_);
+    EXPECT_EQ(0, pref2_cleared_);
+    EXPECT_EQ(1, pref1_initialized_);
+    EXPECT_EQ(0, pref2_initialized_);
+    EXPECT_EQ(0, pref1_unchanged_);
+    EXPECT_EQ(1, pref2_unchanged_);
+
+    // Change the pref. This should be observed by the PrefMetricsService, which
+    // will update the hash in local_state_ to stay in sync.
+    prefs_->SetString(kTrackedPrefs[0], "bar");
+  }
+  // The next startup should record no changes.
+  {
+    scoped_ptr<PrefMetricsService> test = CreatePrefMetricsService();
+    UpdateHistogramSamples();
+    EXPECT_EQ(0, pref1_changed_);
+    EXPECT_EQ(0, pref2_changed_);
+    EXPECT_EQ(0, pref1_cleared_);
+    EXPECT_EQ(0, pref2_cleared_);
+    EXPECT_EQ(0, pref1_initialized_);
+    EXPECT_EQ(0, pref2_initialized_);
+    EXPECT_EQ(1, pref1_unchanged_);
+    EXPECT_EQ(1, pref2_unchanged_);
+  }
+}
+
+TEST_F(PrefMetricsServiceTest, ChangedUserPref) {
+  // Local state is empty. Set a value for the tracked pref. We should record
+  // that we checked preferences once and initialized a hash for the pref.
+  prefs_->SetString(kTrackedPrefs[0], "foo");
+  {
+    scoped_ptr<PrefMetricsService> test = CreatePrefMetricsService();
+    UpdateHistogramSamples();
+    EXPECT_EQ(0, pref1_changed_);
+    EXPECT_EQ(0, pref2_changed_);
+    EXPECT_EQ(0, pref1_cleared_);
+    EXPECT_EQ(0, pref2_cleared_);
+    EXPECT_EQ(1, pref1_initialized_);
+    EXPECT_EQ(0, pref2_initialized_);
+    EXPECT_EQ(0, pref1_unchanged_);
+    EXPECT_EQ(1, pref2_unchanged_);
+    // Hashed prefs should now be stored in local state.
+  }
+  // Change the value of the tracked pref while there is no PrefMetricsService
+  // to update the hash. We should observe a pref value change.
+  prefs_->SetString(kTrackedPrefs[0], "bar");
+  {
+    scoped_ptr<PrefMetricsService> test = CreatePrefMetricsService();
+    UpdateHistogramSamples();
+    EXPECT_EQ(1, pref1_changed_);
+    EXPECT_EQ(0, pref2_changed_);
+    EXPECT_EQ(0, pref1_cleared_);
+    EXPECT_EQ(0, pref2_cleared_);
+    EXPECT_EQ(0, pref1_initialized_);
+    EXPECT_EQ(0, pref2_initialized_);
+    EXPECT_EQ(0, pref1_unchanged_);
+    EXPECT_EQ(1, pref2_unchanged_);
+  }
+  // Clear the value of the tracked pref while there is no PrefMetricsService
+  // to update the hash. We should observe a pref value removal.
+  prefs_->ClearPref(kTrackedPrefs[0]);
+  {
+    scoped_ptr<PrefMetricsService> test = CreatePrefMetricsService();
+    UpdateHistogramSamples();
+    EXPECT_EQ(0, pref1_changed_);
+    EXPECT_EQ(0, pref2_changed_);
+    EXPECT_EQ(1, pref1_cleared_);
+    EXPECT_EQ(0, pref2_cleared_);
+    EXPECT_EQ(0, pref1_initialized_);
+    EXPECT_EQ(0, pref2_initialized_);
+    EXPECT_EQ(0, pref1_unchanged_);
+    EXPECT_EQ(1, pref2_unchanged_);
+  }
+}
+
+// Tests that serialization of dictionary values is stable. If the order of
+// the entries or any whitespace changes, it would cause a spike in pref change
+// UMA events as every hash would change.
+TEST_F(PrefMetricsServiceTest, PrefHashStability) {
+  scoped_ptr<PrefMetricsService> test = CreatePrefMetricsService();
+
+  DictionaryValue dict;
+  dict.Set("a", new StringValue("foo"));
+  dict.Set("d", new StringValue("bad"));
+  dict.Set("b", new StringValue("bar"));
+  dict.Set("c", new StringValue("baz"));
+  EXPECT_EQ("C503FB7C65EEFD5C07185F616A0AA67923C069909933F362022B1F187E73E9A2",
+            GetHashedPrefValue(test.get(), "pref.path1", &dict));
+
+  ListValue list;
+  list.Set(0, new base::FundamentalValue(true));
+  list.Set(1, new base::FundamentalValue(100));
+  list.Set(2, new base::FundamentalValue(1.0));
+  EXPECT_EQ("3163EC3C96263143AF83EA5C9860DFB960EE2263413C7D7D8A9973FCC00E7692",
+            GetHashedPrefValue(test.get(), "pref.path2", &list));
+}
diff --git a/chrome/chrome_tests_unit.gypi b/chrome/chrome_tests_unit.gypi
index 78d760b..be0656f 100644
--- a/chrome/chrome_tests_unit.gypi
+++ b/chrome/chrome_tests_unit.gypi
@@ -1098,6 +1098,7 @@
         'browser/prefs/chrome_pref_service_unittest.cc',
         'browser/prefs/command_line_pref_store_unittest.cc',
         'browser/prefs/incognito_mode_prefs_unittest.cc',
+        'browser/prefs/pref_metrics_service_unittest.cc',
         'browser/prefs/pref_model_associator_unittest.cc',
         'browser/prefs/proxy_config_dictionary_unittest.cc',
         'browser/prefs/proxy_policy_unittest.cc',
diff --git a/chrome/common/pref_names.cc b/chrome/common/pref_names.cc
index d719e4b..c111cb5 100644
--- a/chrome/common/pref_names.cc
+++ b/chrome/common/pref_names.cc
@@ -2597,4 +2597,9 @@ const char kEnableDRM[] = "settings.privacy.drm_enabled";
 const char kWatchdogExtensionActive[] =
     "profile.extensions.activity_log.watchdog_extension_active";
 
+// A dictionary pref which maps profile names to dictionary values which hold
+// hashes of profile prefs that we track to detect changes that happen outside
+// of Chrome.
+const char kProfilePreferenceHashes[] = "profile.preference_hashes";
+
 }  // namespace prefs
diff --git a/chrome/common/pref_names.h b/chrome/common/pref_names.h
index 2be5bda..ccc0e2e 100644
--- a/chrome/common/pref_names.h
+++ b/chrome/common/pref_names.h
@@ -949,6 +949,8 @@ extern const char kEnableDRM[];
 
 extern const char kWatchdogExtensionActive[];
 
+extern const char kProfilePreferenceHashes[];
+
 }  // namespace prefs
 
 #endif  // CHROME_COMMON_PREF_NAMES_H_
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index f2ddfb1..2d1fec4 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -14387,6 +14387,36 @@ other types of suffix sets.
   </summary>
 </histogram>
 
+<histogram name="Settings.TrackedPreferenceChanged" enum="TrackedPreference">
+  <summary>
+    The id of a tracked preference whose value has been changed since the last
+    time Chrome set it.
+  </summary>
+</histogram>
+
+<histogram name="Settings.TrackedPreferenceCleared" enum="TrackedPreference">
+  <summary>
+    The id of a tracked preference whose value has been cleared since the last
+    time Chrome set it.
+  </summary>
+</histogram>
+
+<histogram name="Settings.TrackedPreferenceInitialized"
+    enum="TrackedPreference">
+  <summary>
+    The id of a tracked preference whose last value isn't known. We may be just
+    starting to track the preference, or local state may have been changed
+    outside of Chrome.
+  </summary>
+</histogram>
+
+<histogram name="Settings.TrackedPreferenceUnchanged" enum="TrackedPreference">
+  <summary>
+    The id of a tracked preference whose value has not changed since the last
+    time Chrome set it.
+  </summary>
+</histogram>
+
 <histogram name="SimpleCache.App.CheckCRCResult" enum="CheckCRCResult">
   <summary>
     Whether or not the CRC was checked at the moment when the last reference to
@@ -25196,6 +25226,21 @@ other types of suffix sets.
   </int>
 </enum>
 
+<enum name="TrackedPreference" type="int">
+  <int value="0" label="prefs::kShowHomeButton"/>
+  <int value="1" label="prefs::kHomePageIsNewTabPage"/>
+  <int value="2" label="prefs::kHomePage"/>
+  <int value="3" label="prefs::kRestoreOnStartup"/>
+  <int value="4" label="prefs::kURLsToRestoreOnStartup"/>
+  <int value="5" label="prefs::kExtensionsPref"/>
+  <int value="6" label="prefs::kGoogleServicesLastUsername"/>
+  <int value="7" label="prefs::kSearchProviderOverrides"/>
+  <int value="8" label="prefs::kDefaultSearchProviderSearchURL"/>
+  <int value="9" label="prefs::kDefaultSearchProviderKeyword"/>
+  <int value="10" label="prefs::kDefaultSearchProviderName"/>
+  <int value="11" label="prefs::kPinnedTabs"/>
+</enum>
+
 <enum name="TranslateError" type="int">
   <int value="0" label="No error"/>
   <int value="1" label="Network error"/>