Extended proxy abstraction, to call certain methods to the worker thread.

webrtc/api/videotrack.h

 /*
  *  Copyright 2012 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 29 matching lines @@
   virtual std::string kind() const;
 
  protected:
   VideoTrack(const std::string& id, VideoTrackSourceInterface* video_source);
   ~VideoTrack();
 
  private:
   // Implements ObserverInterface. Observes |video_source_| state.
   void OnChanged() override;
 
-  rtc::ThreadChecker thread_checker_;
+  rtc::ThreadChecker signaling_thread_checker_;
+  rtc::ThreadChecker worker_thread_checker_;
   rtc::scoped_refptr<VideoTrackSourceInterface> video_source_;
 };
 
 }  // namespace webrtc
 
 #endif  // WEBRTC_API_VIDEOTRACK_H_