Use the browser UI thread for audio on OSX.

media/audio/audio_manager.h

Index: media/audio/audio_manager.h
diff --git a/media/audio/audio_manager.h b/media/audio/audio_manager.h
index a20af4a7d6a7a174ac28b6042c193bc2b70c37a9..e924096d13b1d30785682f9877cfe781a74e49c1 100644
--- a/media/audio/audio_manager.h
+++ b/media/audio/audio_manager.h
@@ -112,6 +112,11 @@ class MEDIA_EXPORT AudioManager {
   // Returns message loop used for audio IO.
   virtual scoped_refptr<base::MessageLoopProxy> GetMessageLoop() = 0;
 
+  // Heavyweight tasks should use GetWorkerLoop() instead of GetMessageLoop().
+  // On most platforms they are the same, but some share the UI loop with the
+  // audio IO loop.
+  virtual scoped_refptr<base::MessageLoopProxy> GetWorkerLoop() = 0;
+
   // Allows clients to listen for device state changes; e.g. preferred sample
   // rate or channel layout changes.  The typical response to receiving this
   // callback is to recreate the stream.