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

webrtc/modules/video_coding/generic_decoder.cc

 /*
  *  Copyright (c) 2012 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 55 matching lines @@
   }
 
   const int64_t now_ms = _clock->TimeInMilliseconds();
   if (decode_time_ms < 0) {
     decode_time_ms =
         static_cast<int32_t>(now_ms - frameInfo->decodeStartTimeMs);
   }
   _timing->StopDecodeTimer(decodedImage.timestamp(), decode_time_ms, now_ms,
                            frameInfo->renderTimeMs);
 
   if (callback != NULL) {

[stefan-webrtc, 2016/09/26 10:44:08]

Can we possibly instead DCHECK on callback != nullptr?

[sakal, 2016/09/26 11:23:59]

_receiveCallback is initiated with nullptr in the constructor and is only set
after SetUserReceiveCallback. Since it is not trivial to prove this will never
happen I would err on the side of caution. The if statement was added for a
reason probably.

[stefan-webrtc, 2016/09/26 11:39:22]

It's always set here:
https://cs.chromium.org/chromium/src/third_party/webrtc/video/video_stream_de...

     decodedImage.set_render_time_ms(frameInfo->renderTimeMs);
     decodedImage.set_rotation(frameInfo->rotation);
     callback->FrameToRender(decodedImage);
+  } else {
+    LOG(LS_INFO) << "Dropping frame because callback is NULL.";
   }
   return WEBRTC_VIDEO_CODEC_OK;
 }
 
 int32_t VCMDecodedFrameCallback::ReceivedDecodedReferenceFrame(
     const uint64_t pictureId) {
   CriticalSectionScoped cs(_critSect);
   if (_receiveCallback != NULL) {
     return _receiveCallback->ReceivedDecodedReferenceFrame(pictureId);
   }
@@ skipping 89 matching lines @@
 
 bool VCMGenericDecoder::External() const {
   return _isExternal;
 }
 
 bool VCMGenericDecoder::PrefersLateDecoding() const {
   return _decoder->PrefersLateDecoding();
 }
 
 }  // namespace webrtc