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

webrtc/media/base/videoframe.h

Index: webrtc/media/base/videoframe.h
diff --git a/webrtc/media/base/videoframe.h b/webrtc/media/base/videoframe.h
index 3d765ad5df4ec652550ccbd2877f30ce95ac4b53..a1393499f9e72742ba5792e790c09cf5fe52baf5 100644
--- a/webrtc/media/base/videoframe.h
+++ b/webrtc/media/base/videoframe.h
@@ -38,6 +38,9 @@ class VideoFrame {
   virtual const rtc::scoped_refptr<webrtc::VideoFrameBuffer>&
   video_frame_buffer() const = 0;
 
+  // Frame ID. Normally RTP timestamp when the frame was received using RTP.
+  virtual uint32_t transport_frame_id() const = 0;
+
   // System monotonic clock, same timebase as rtc::TimeMicros().
   virtual int64_t timestamp_us() const = 0;
   virtual void set_timestamp_us(int64_t time_us) = 0;