Use the browser UI thread for audio on OSX.

media/audio/audio_manager_base.h

Index: media/audio/audio_manager_base.h
diff --git a/media/audio/audio_manager_base.h b/media/audio/audio_manager_base.h
index 3d75b77e1ef31fede762138b867ff22818b8f178..d6b40228754166a3d544da33ca6b1a917a08eafb 100644
--- a/media/audio/audio_manager_base.h
+++ b/media/audio/audio_manager_base.h
@@ -39,6 +39,7 @@ class MEDIA_EXPORT AudioManagerBase : public AudioManager {
   virtual ~AudioManagerBase();
 
   virtual scoped_refptr<base::MessageLoopProxy> GetMessageLoop() OVERRIDE;
+  virtual scoped_refptr<base::MessageLoopProxy> GetWorkerLoop() OVERRIDE;
 
   virtual string16 GetAudioInputDeviceModel() OVERRIDE;
 
@@ -164,6 +165,7 @@ class MEDIA_EXPORT AudioManagerBase : public AudioManager {
   // tasks which run on the audio thread even after Shutdown() has been started
   // and GetMessageLoop() starts returning NULL.
   scoped_refptr<base::MessageLoopProxy> message_loop_;
+  scoped_refptr<base::MessageLoopProxy> worker_loop_;
 
   DISALLOW_COPY_AND_ASSIGN(AudioManagerBase);
 };