RTCMediaStreamTrackStats.framesDropped collected by RTCStatsCollector.

webrtc/pc/rtcstatscollector.cc

Index: webrtc/pc/rtcstatscollector.cc
diff --git a/webrtc/pc/rtcstatscollector.cc b/webrtc/pc/rtcstatscollector.cc
index 1a25f32a9cd9e3b59d740aa220d5634e7cd9fec0..774d1d73599a9fc0a6f87b58152dadce0f14198d 100644
--- a/webrtc/pc/rtcstatscollector.cc
+++ b/webrtc/pc/rtcstatscollector.cc
@@ -445,6 +445,10 @@ ProduceMediaStreamTrackStatsFromVideoReceiverInfo(
   // received from. Since we don't support that, this is correct and is the same
   // value as "RTCInboundRTPStreamStats.framesDecoded". crbug.com/659137
   video_track_stats->frames_decoded = video_receiver_info.frames_decoded;
+  RTC_DCHECK_GE(video_receiver_info.frames_received,
+                video_receiver_info.frames_rendered);
+  video_track_stats->frames_dropped = video_receiver_info.frames_received -
+                                      video_receiver_info.frames_rendered;
   return video_track_stats;
 }