Use the browser UI thread for audio on OSX.

media/audio/mock_audio_manager.cc

Index: media/audio/mock_audio_manager.cc
diff --git a/media/audio/mock_audio_manager.cc b/media/audio/mock_audio_manager.cc
index 49bc8d1bb5f9bfef65f0bd4e2f4f333e76414eed..9d74beba20e9f84df67127607e4aa96517a0240a 100644
--- a/media/audio/mock_audio_manager.cc
+++ b/media/audio/mock_audio_manager.cc
@@ -65,6 +65,10 @@ scoped_refptr<base::MessageLoopProxy> MockAudioManager::GetMessageLoop() {
   return message_loop_proxy_;
 }
 
+scoped_refptr<base::MessageLoopProxy> MockAudioManager::GetWorkerLoop() {
+  return message_loop_proxy_;
+}
+
 void MockAudioManager::AddOutputDeviceChangeListener(
     AudioDeviceListener* listener) {
 }