Add histogram stats for AV sync stream offset:

webrtc/video/receive_statistics_proxy.cc

Index: webrtc/video/receive_statistics_proxy.cc
diff --git a/webrtc/video/receive_statistics_proxy.cc b/webrtc/video/receive_statistics_proxy.cc
index c92d4083c04208ba7d1096ce8f5d74a7cd84343f..b5e9ab0734dcb9b39ffa975f6f7eb7201cd38fab 100644
--- a/webrtc/video/receive_statistics_proxy.cc
+++ b/webrtc/video/receive_statistics_proxy.cc
@@ -59,6 +59,10 @@ void ReceiveStatisticsProxy::UpdateHistograms() {
     RTC_HISTOGRAM_COUNTS_10000("WebRTC.Video.ReceivedWidthInPixels", width);
     RTC_HISTOGRAM_COUNTS_10000("WebRTC.Video.ReceivedHeightInPixels", height);
   }
+  int sync_offset_ms = sync_offset_counter_.Avg(kMinRequiredSamples);
+  if (sync_offset_ms != -1)
+    RTC_HISTOGRAM_COUNTS_10000("WebRTC.Video.AVSyncOffsetInMs", sync_offset_ms);
+
   int qp = qp_counters_.vp8.Avg(kMinRequiredSamples);
   if (qp != -1)
     RTC_HISTOGRAM_COUNTS_200("WebRTC.Video.Decoded.Vp8.Qp", qp);
@@ -239,6 +243,12 @@ void ReceiveStatisticsProxy::OnRenderedFrame(const VideoFrame& frame) {
   }
 }
 
+void ReceiveStatisticsProxy::OnSyncOffsetUpdated(int64_t sync_offset_ms) {
+  rtc::CritScope lock(&crit_);
+  sync_offset_counter_.Add(std::abs(sync_offset_ms));
+  stats_.sync_offset_ms = sync_offset_ms;
+}
+
 void ReceiveStatisticsProxy::OnReceiveRatesUpdated(uint32_t bitRate,
                                                    uint32_t frameRate) {
 }