Cleanup AudioDeviceThread to reduce locking and unused features.

media/audio/audio_output_device.h

Index: media/audio/audio_output_device.h
diff --git a/media/audio/audio_output_device.h b/media/audio/audio_output_device.h
index c0faacd11517fab6c80e168a0c433b627dfb5561..d96c15fbdb4b8d9bf2bd9a50ef4975883337a729 100644
--- a/media/audio/audio_output_device.h
+++ b/media/audio/audio_output_device.h
@@ -198,8 +198,8 @@ class MEDIA_EXPORT AudioOutputDevice
   // In order to avoid a race between OnStreamCreated and Stop(), we use this
   // guard to control stopping and starting the audio thread.
   base::Lock audio_thread_lock_;
-  AudioDeviceThread audio_thread_;
   std::unique_ptr<AudioOutputDevice::AudioThreadCallback> audio_callback_;
+  std::unique_ptr<AudioDeviceThread> audio_thread_;
 
   // Temporary hack to ignore OnStreamCreated() due to the user calling Stop()
   // so we don't start the audio thread pointing to a potentially freed