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

content/renderer/media/mock_media_stream_video_sink.cc

Index: content/renderer/media/mock_media_stream_video_sink.cc
diff --git a/content/renderer/media/mock_media_stream_video_sink.cc b/content/renderer/media/mock_media_stream_video_sink.cc
index 347ca62e728bcff25d0e0a398ee22373a8b91c69..7a054ab3f715ff1d3ee22422a2470ca0c0f5a22a 100644
--- a/content/renderer/media/mock_media_stream_video_sink.cc
+++ b/content/renderer/media/mock_media_stream_video_sink.cc
@@ -31,6 +31,7 @@ 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();