Refactor VideoTracks to forward all sinks to its source

webrtc/media/base/videosourceinterface.h

 /*
  *  Copyright (c) 2016 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.
  */
 
 #ifndef WEBRTC_MEDIA_BASE_VIDEOSOURCEINTERFACE_H_
 #define WEBRTC_MEDIA_BASE_VIDEOSOURCEINTERFACE_H_
 
 #include "webrtc/media/base/videosinkinterface.h"
 #include "webrtc/base/optional.h"
 
 namespace rtc {
 
 // VideoSinkWants is used for notifying the source of properties a video frame
 // should have when it is delivered to a certain sink.
 struct VideoSinkWants {
   // Tells the source whether the sink wants frames with rotation applied.
   // By default, the rotation is applied by the source.
   bool rotation_applied = true;
 
+  // Tells the source that the sink only wants black frames.
+  bool black_frames = false;
+
   // Tells the source the maximum number of pixels the sink wants.
   rtc::Optional<int> max_pixel_count;
   // Like |max_pixel_count| but relative to the given value. The source is
   // requested to produce frames with a resolution one "step up" from the given
   // value. In practice, this means that the sink can consume this amount of
   // pixels but wants more and the source should produce a resolution one
   // "step" higher than this but not higher.
   rtc::Optional<int> max_pixel_count_step_up;
 };
 
 template <typename VideoFrameT>
 class VideoSourceInterface {
  public:
   virtual void AddOrUpdateSink(VideoSinkInterface<VideoFrameT>* sink,
                                const VideoSinkWants& wants) = 0;
   // RemoveSink must guarantee that at the time the method returns,
   // there is no current and no future calls to VideoSinkInterface::OnFrame.
   virtual void RemoveSink(VideoSinkInterface<VideoFrameT>* sink) = 0;
 
  protected:
   virtual ~VideoSourceInterface() {}
 };
 
 }  // namespace rtc
 #endif  // WEBRTC_MEDIA_BASE_VIDEOSOURCEINTERFACE_H_