AudioManagerBase: Create and run the audio thread lazily.

media/audio/mac/audio_manager_mac.h

Index: media/audio/mac/audio_manager_mac.h
diff --git a/media/audio/mac/audio_manager_mac.h b/media/audio/mac/audio_manager_mac.h
index 2112a88308151cae0e54e1590d8ced71c2a3899c..0545f43754be27d69f14c87b5bc212159d98051b 100644
--- a/media/audio/mac/audio_manager_mac.h
+++ b/media/audio/mac/audio_manager_mac.h
@@ -29,6 +29,8 @@ class MEDIA_EXPORT AudioManagerMac : public AudioManagerBase {
   AudioManagerMac(AudioLogFactory* audio_log_factory);
 
   // Implementation of AudioManager.
+  scoped_refptr<base::SingleThreadTaskRunner> GetTaskRunner() override;
+  scoped_refptr<base::SingleThreadTaskRunner> GetWorkerTaskRunner() override;
   bool HasAudioOutputDevices() override;
   bool HasAudioInputDevices() override;
   void GetAudioInputDeviceNames(AudioDeviceNames* device_names) override;
@@ -122,6 +124,9 @@ class MEDIA_EXPORT AudioManagerMac : public AudioManagerBase {
 
   scoped_ptr<AudioDeviceListenerMac> output_device_listener_;
 
+  scoped_refptr<base::SingleThreadTaskRunner> task_runner_;
+  scoped_ptr<base::Thread> worker_thread_;
+
   // Track the output sample-rate and the default output device
   // so we can intelligently handle device notifications only when necessary.
   int current_sample_rate_;