Change a disabled MediaStreamVideoTrack to output black video frames for each incoming frame.

content/renderer/media/mock_media_stream_video_sink.cc

 // Copyright 2014 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.
 
 #include "content/renderer/media/mock_media_stream_video_sink.h"
 
 #include "media/base/bind_to_current_loop.h"
 
 namespace content {
 
@@ skipping 13 matching lines @@
   return media::BindToCurrentLoop(
       base::Bind(
           &MockMediaStreamVideoSink::DeliverVideoFrame,
           weak_factory_.GetWeakPtr()));
 }
 
 void MockMediaStreamVideoSink::DeliverVideoFrame(
     const scoped_refptr<media::VideoFrame>& frame,
     const media::VideoCaptureFormat& format,
     const base::TimeTicks& estimated_capture_time) {
+  last_frame_ = frame;
   ++number_of_frames_;
   format_ = frame->format();
   frame_size_ = frame->natural_size();
   OnVideoFrame();
 }
 
 void MockMediaStreamVideoSink::OnReadyStateChanged(
     blink::WebMediaStreamSource::ReadyState state) {
   state_ = state;
 }
 
 void MockMediaStreamVideoSink::OnEnabledChanged(bool enabled) {
   enabled_ = enabled;
 }
 
 }  // namespace content