RTCMediaStreamTrackStats.framesReceived collected by RTCStatsCollector.

VideoReceiverInfo::frames_received added based on
VideoReceiveStream::Stats::frame_counts (.key_frames + .delta_frames).

BUG=webrtc:6757, chromium:659137, chromium:627816

Review-Url: https://codereview.webrtc.org/2607913002
Cr-Commit-Position: refs/heads/master@{#16185}
diff --git a/webrtc/api/rtcstatscollector.cc b/webrtc/api/rtcstatscollector.cc
index 216a5ac..5d78273 100644
--- a/webrtc/api/rtcstatscollector.cc
+++ b/webrtc/api/rtcstatscollector.cc
@@ -436,6 +436,7 @@
     video_track_stats->frame_height = static_cast<uint32_t>(
         video_receiver_info.frame_height);
   }
+  video_track_stats->frames_received = video_receiver_info.frames_received;
   return video_track_stats;
 }