Add cricket::VideoFrame::frame_id() and set it to RTP timestamp.

webrtc/media/base/videobroadcaster.cc

Index: webrtc/media/base/videobroadcaster.cc
diff --git a/webrtc/media/base/videobroadcaster.cc b/webrtc/media/base/videobroadcaster.cc
index ef5c49b4a188a416641bd2af7ea04885f31842d5..69350c19524784725e5e8e0ceebe59cdecd5e511 100644
--- a/webrtc/media/base/videobroadcaster.cc
+++ b/webrtc/media/base/videobroadcaster.cc
@@ -55,8 +55,8 @@ void VideoBroadcaster::OnFrame(const cricket::VideoFrame& frame) {
   for (auto& sink_pair : sink_pairs()) {
     if (sink_pair.wants.black_frames) {
       sink_pair.sink->OnFrame(cricket::WebRtcVideoFrame(
-          GetBlackFrameBuffer(frame.width(), frame.height()),
-          frame.rotation(), frame.timestamp_us()));
+          GetBlackFrameBuffer(frame.width(), frame.height()), frame.rotation(),
+          frame.timestamp_us(), frame.frame_id()));
     } else {
       sink_pair.sink->OnFrame(frame);
     }