Fix the way locks are acquired in WebRtcAudioDeviceImpl::SetAudioRenderer.

content/renderer/media/webrtc_audio_device_impl.h

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CONTENT_RENDERER_MEDIA_WEBRTC_AUDIO_DEVICE_IMPL_H_
 #define CONTENT_RENDERER_MEDIA_WEBRTC_AUDIO_DEVICE_IMPL_H_
 
 #include <string>
 #include <vector>
 
@@ skipping 149 matching lines @@
 // steady state is that the volume control reaches its max and the new_volume
 // value from the AGC is zero. A loud voice input is required to break this
 // state and start lowering the level again.
 //
 // Implementation notes:
 //
 //  - This class must be created and destroyed on the main render thread and
 //    most methods are called on the same thread. However, some methods are
 //    also called on a Libjingle worker thread. RenderData is called on the
 //    AudioOutputDevice thread and CaptureData on the AudioInputDevice thread.
-//    To summarize: this class lives on four different threads.
+//    To summarize: this class lives on four different threads, so it is
+//    important to be careful with the order in which locks are acquired in
+//    order to avoid potential deadlocks.
 //  - The webrtc::AudioDeviceModule is reference counted.
 //  - AGC is only supported in combination with the WASAPI-based audio layer
 //    on Windows, i.e., it is not supported on Windows XP.
 //  - All volume levels required for the AGC scheme are transfered in a
 //    normalized range [0.0, 1.0]. Scaling takes place in both endpoints
 //    (WebRTC client a media layer). This approach ensures that we can avoid
 //    transferring maximum levels between the renderer and the browser.
 //
 
 namespace content {
@@ skipping 213 matching lines @@
   // Buffer used for temporary storage during render callback.
   // It is only accessed by the audio render thread.
   std::vector<int16> render_buffer_;
 
   DISALLOW_COPY_AND_ASSIGN(WebRtcAudioDeviceImpl);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_RENDERER_MEDIA_WEBRTC_AUDIO_DEVICE_IMPL_H_