Add dropped frames metric on the receive side

Reported to UMA and logged for at the end of the call.

Bug: webrtc:8355
Change-Id: I4ef31bf9e55feaba9cf28be5cb4fcfae929c7179
Reviewed-on: https://webrtc-review.googlesource.com/53760
Reviewed-by: Philip Eliasson <philipel@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Commit-Queue: Ilya Nikolaevskiy <ilnik@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22132}
diff --git a/video/rtp_video_stream_receiver.cc b/video/rtp_video_stream_receiver.cc
index 7a06f1c..e01e85f 100644
--- a/video/rtp_video_stream_receiver.cc
+++ b/video/rtp_video_stream_receiver.cc
@@ -589,6 +589,10 @@
                                                : RtcpMode::kOff);
 }
 
+int RtpVideoStreamReceiver::GetUniqueFramesSeen() const {
+  return packet_buffer_->GetUniqueFramesSeen();
+}
+
 void RtpVideoStreamReceiver::StartReceive() {
   RTC_DCHECK_CALLED_SEQUENTIALLY(&worker_task_checker_);
   receiving_ = true;