Fix threading issues in the audio pipeline for WebRTC remoting protocol.

remoting/protocol/webrtc_audio_sink_adapter.h

Index: remoting/protocol/webrtc_audio_sink_adapter.h
diff --git a/remoting/protocol/webrtc_audio_sink_adapter.h b/remoting/protocol/webrtc_audio_sink_adapter.h
index 4f4969a1809f61c77142efcd51fdeb962216fb3c..aab3f714e3b4404ff4ed9523b11eff2c2b0a2aa7 100644
--- a/remoting/protocol/webrtc_audio_sink_adapter.h
+++ b/remoting/protocol/webrtc_audio_sink_adapter.h
@@ -7,8 +7,13 @@
 
 #include "base/memory/ref_counted.h"
 #include "base/memory/weak_ptr.h"
+#include "base/threading/thread_task_runner_handle.h"
 #include "third_party/webrtc/api/mediastreaminterface.h"
 
+namespace base {
+class SingleThreadTaskRunner;
+}  // namespace base
+
 namespace remoting {
 namespace protocol {
 
@@ -27,10 +32,10 @@ class WebrtcAudioSinkAdapter : public webrtc::AudioTrackSinkInterface {
               size_t number_of_frames) override;
 
  private:
+  scoped_refptr<base::SingleThreadTaskRunner> task_runner_;
+  base::WeakPtr<AudioStub> audio_stub_;
   scoped_refptr<webrtc::MediaStreamInterface> media_stream_;
   scoped_refptr<webrtc::AudioTrackInterface> audio_track_;
-
-  base::WeakPtr<AudioStub> audio_stub_;
 };
 
 }  // namespace protocol