Reland of New method I420Buffer::SetToBlack.

webrtc/media/base/videobroadcaster.h

Index: webrtc/media/base/videobroadcaster.h
diff --git a/webrtc/media/base/videobroadcaster.h b/webrtc/media/base/videobroadcaster.h
index 764c749b0c3f1dfbf29edf61f35da24eeb84e20b..5d4e2ae658480e0ff86e71121e54e6c180c57705 100644
--- a/webrtc/media/base/videobroadcaster.h
+++ b/webrtc/media/base/videobroadcaster.h
@@ -49,14 +49,15 @@ class VideoBroadcaster : public VideoSourceBase,
 
  protected:
   void UpdateWants() EXCLUSIVE_LOCKS_REQUIRED(sinks_and_wants_lock_);
-  const cricket::VideoFrame& GetBlackFrame(const cricket::VideoFrame& frame)
+  const rtc::scoped_refptr<webrtc::VideoFrameBuffer>& GetBlackFrameBuffer(
+      int width, int height)
       EXCLUSIVE_LOCKS_REQUIRED(sinks_and_wants_lock_);
 
   ThreadChecker thread_checker_;
   rtc::CriticalSection sinks_and_wants_lock_;
 
   VideoSinkWants current_wants_ GUARDED_BY(sinks_and_wants_lock_);
-  std::unique_ptr<cricket::WebRtcVideoFrame> black_frame_;
+  rtc::scoped_refptr<webrtc::VideoFrameBuffer> black_frame_buffer_;
 };
 
 }  // namespace rtc