Mac AVFoundation: Enumerate devices in device/audio thread.

Bug http://crbug.com/359589 describes a Chrome hang
where two threads are calling [AVCaptureDeviceGlue devices] 
at the same time to enumerate the devices. The first call 
comes from DeviceMonitorMac::StartMonitoring() on UI
thread while the second call comes from the Audio Thread,
a.k.a. Device Thread on
+VideoCaptureDeviceAVFoundation::getDeviceNames:.

DMM == DeviceMonitorMac

The solution proposed here is to do as much as possible
on Device thread :
- when there is a device change via DMM::OnDeviceChanged(),
enumerate on Device Thread, then consolidate the device
list on UI thread.
- First |suspend_observer_| registration happens on Device 
Thread. This suspend observer lives in device thread.
- Observer removal happens dutifully on Device Thread
destructor in -CrAVFoundationDeviceObserver::dealloc

BUG=288562, 359589

Review URL: https://codereview.chromium.org/234563004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@265254 0039d316-1c4b-4281-b951-d872f2087c98

1 files changed, 5 insertions, 3 deletions

diff --git a/content/browser/device_monitor_mac.h b/content/browser/device_monitor_mac.h
index 696b961..b3d3f9f 100644
--- a/content/browser/device_monitor_mac.h
+++ b/content/browser/device_monitor_mac.h
@@ -25,8 +25,10 @@ class 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();
+  // OS supports it. The |device_task_runner| argument represents the thread on
+  // which device enumeration will occur.
+  void StartMonitoring(
+    const scoped_refptr<base::SingleThreadTaskRunner>& device_task_runner);
 
   // Method called by the internal DeviceMonitorMacImpl object
   // |device_monitor_impl_| when a device of type |type| has been added to or
@@ -38,7 +40,7 @@ class DeviceMonitorMac {
   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.
+  // are called in the correct thread, the UI thread, that also owns the object.
   base::ThreadChecker thread_checker_;
 
   DISALLOW_COPY_AND_ASSIGN(DeviceMonitorMac);