Mac AVFoundation/QTKit: delay DeviceMonitorMac startup to first GetUserMedia.

Currently DeviceMonitorMac is constructed from BrowserMainLoop in the
first compasses of Chrome bringup. DeviceMonitorMac creates either a
QTKitMonitorImpl or a AVFoundationMonitorImpl, that immediately
starts observing the system capture devices. This monitoring is
not needed until the user actively starts operating the devices,
that happens in MediaStreamManager.

This CL adds a method StartMonitoring() to DeviceMonitorMac that is
exercised from MediaStreamManager::StartMonitoring().

The performance regression in bugs 349616 and 348020 affects only
AVFoundation, but both implementations would benefit from a delayed
startup.

Tested locally via using --[enable/disable]-avfoundation, connecting
and disconnecting devices etc.

BUG=288562, 349616, 348020

Review URL: https://codereview.chromium.org/189443010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256483 0039d316-1c4b-4281-b951-d872f2087c98

1 files changed, 10 insertions, 0 deletions

diff --git a/content/browser/device_monitor_mac.h b/content/browser/device_monitor_mac.h
index 6def493..696b961 100644
--- a/content/browser/device_monitor_mac.h
+++ b/content/browser/device_monitor_mac.h
@@ -7,6 +7,7 @@
 
 #include "base/basictypes.h"
 #include "base/system_monitor/system_monitor.h"
+#include "base/threading/thread_checker.h"
 
 namespace {
 class DeviceMonitorMacImpl;
@@ -22,6 +23,11 @@ class DeviceMonitorMac {
   DeviceMonitorMac();
   ~DeviceMonitorMac();
 
+  // Registers the observers for the audio/video device removal, connection and
+  // suspension. The AVFoundation library is also loaded and initialised if the
+  // OS supports it.
+  void StartMonitoring();
+
   // Method called by the internal DeviceMonitorMacImpl object
   // |device_monitor_impl_| when a device of type |type| has been added to or
   // removed from the system. This code executes in the notification thread
@@ -31,6 +37,10 @@ class DeviceMonitorMac {
  private:
   scoped_ptr<DeviceMonitorMacImpl> device_monitor_impl_;
 
+  // |thread_checker_| is used to check that constructor and StartMonitoring()
+  // are called in the correct thread, that also owns the object.
+  base::ThreadChecker thread_checker_;
+
   DISALLOW_COPY_AND_ASSIGN(DeviceMonitorMac);
 };