Add histogram stats for AV sync stream offset:

webrtc/video_receive_stream.h

Index: webrtc/video_receive_stream.h
diff --git a/webrtc/video_receive_stream.h b/webrtc/video_receive_stream.h
index 51d842616b467ff83da460aa46d1e8d60fbc8f59..249523c29b1e142b0ba03d0018f4effa70fd9f54 100644
--- a/webrtc/video_receive_stream.h
+++ b/webrtc/video_receive_stream.h
@@ -11,6 +11,7 @@
 #ifndef WEBRTC_VIDEO_RECEIVE_STREAM_H_
 #define WEBRTC_VIDEO_RECEIVE_STREAM_H_
 
+#include <limits>
 #include <map>
 #include <string>
 #include <vector>
@@ -66,6 +67,8 @@ class VideoReceiveStream : public ReceiveStream {
     int total_bitrate_bps = 0;
     int discarded_packets = 0;
 
+    int sync_offset_ms = std::numeric_limits<int>::max();
+
     uint32_t ssrc = 0;
     std::string c_name;
     StreamDataCounters rtp_stats;