Add thread safety to AudioManagerBase to protect access to the audio thread member variable.

media/audio/audio_output_dispatcher.h

Index: media/audio/audio_output_dispatcher.h
diff --git a/media/audio/audio_output_dispatcher.h b/media/audio/audio_output_dispatcher.h
index 08e6893b6b9ffde60ee75be0ee129c26f0c7c5a7..4eb9512bd1ea8e4b14ef6895e4373bbf6fc0dda8 100644
--- a/media/audio/audio_output_dispatcher.h
+++ b/media/audio/audio_output_dispatcher.h
@@ -69,7 +69,10 @@ class MEDIA_EXPORT AudioOutputDispatcher
   // Called on the audio thread when the AudioManager is shutting down.
   void Shutdown();
 
-  MessageLoop* message_loop();
+  // Used to DCHECK correct usage.  The method _should_ always be called
+  // from the audio thread, so we don't have any locks to protect the
+  // message loop pointer.
+  bool CurrentThreadIsAudioThread() const;
 
  private:
   friend class AudioOutputProxyTest;