Add logging statements to places where the frame might be dropped in WebRTC pipeline.

webrtc/video/video_receive_stream.cc

 /*
  *  Copyright (c) 2013 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 270 matching lines @@
   rtc::VideoSinkInterface<VideoFrame>* renderer = nullptr;
   if (config_.renderer) {
     if (config_.disable_prerenderer_smoothing) {
       renderer = this;
     } else {
       incoming_video_stream_.reset(
           new IncomingVideoStream(config_.render_delay_ms, this));
       renderer = incoming_video_stream_.get();
     }
   }
+  RTC_DCHECK(renderer != nullptr);
 
   video_stream_decoder_.reset(new VideoStreamDecoder(
       &video_receiver_, &rtp_stream_receiver_, &rtp_stream_receiver_,
       rtp_stream_receiver_.IsRetransmissionsEnabled(),
       rtp_stream_receiver_.IsFecEnabled(), &stats_proxy_, renderer,
       config_.pre_render_callback));
   // Register the channel to receive stats updates.
   call_stats_->RegisterStatsObserver(video_stream_decoder_.get());
   // Start the decode thread
   decode_thread_.Start();
@@ skipping 95 matching lines @@
     const std::vector<uint16_t>& sequence_numbers) {
   rtp_stream_receiver_.RequestPacketRetransmit(sequence_numbers);
 }
 
 void VideoReceiveStream::RequestKeyFrame() {
   rtp_stream_receiver_.RequestKeyFrame();
 }
 
 }  // namespace internal
 }  // namespace webrtc