Add histogram stats for AV sync stream offset:

webrtc/video/receive_statistics_proxy.h

Index: webrtc/video/receive_statistics_proxy.h
diff --git a/webrtc/video/receive_statistics_proxy.h b/webrtc/video/receive_statistics_proxy.h
index 043a0a2d6527fd597dbf06aa415fcee9387e8aca..f466caa597aa12ba993025523dbed90408385473 100644
--- a/webrtc/video/receive_statistics_proxy.h
+++ b/webrtc/video/receive_statistics_proxy.h
@@ -45,7 +45,7 @@ class ReceiveStatisticsProxy : public VCMReceiveStatisticsCallback,
   VideoReceiveStream::Stats GetStats() const;
 
   void OnDecodedFrame();
-  void OnRenderedFrame(int width, int height);
+  void OnRenderedFrame(int width, int height, int64_t stream_sync_offset_ms);
   void OnIncomingPayloadType(int payload_type);
   void OnDecoderImplementationName(const char* implementation_name);
   void OnIncomingRate(unsigned int framerate, unsigned int bitrate_bps);
@@ -106,6 +106,7 @@ class ReceiveStatisticsProxy : public VCMReceiveStatisticsCallback,
   rtc::RateTracker render_pixel_tracker_ GUARDED_BY(crit_);
   SampleCounter render_width_counter_ GUARDED_BY(crit_);
   SampleCounter render_height_counter_ GUARDED_BY(crit_);
+  SampleCounter render_sync_offset_counter_ GUARDED_BY(crit_);
   SampleCounter decode_time_counter_ GUARDED_BY(crit_);
   SampleCounter delay_counter_ GUARDED_BY(crit_);
   ReportBlockStats report_block_stats_ GUARDED_BY(crit_);