Add histogram stats for AV sync stream offset:

webrtc/video/vie_sync_module.h

Index: webrtc/video/vie_sync_module.h
diff --git a/webrtc/video/vie_sync_module.h b/webrtc/video/vie_sync_module.h
index a5dff437f02c302adc415fac7b154a2b5fbf5730..2b499ff4d38a49cef992281e1af155712ea8f896 100644
--- a/webrtc/video/vie_sync_module.h
+++ b/webrtc/video/vie_sync_module.h
@@ -24,8 +24,10 @@
 
 namespace webrtc {
 
+class Clock;
 class RtpRtcp;
 class VideoCodingModule;
+class VideoFrame;
 class ViEChannel;
 class VoEVideoSync;
 
@@ -43,9 +45,15 @@ class ViESyncModule : public Module {
   int64_t TimeUntilNextProcess() override;
   void Process() override;
 
+  // Gets the sync offset between the current played out audio frame and the
+  // video |frame|. Returns true on success, false otherwise.
+  bool GetStreamSyncOffsetInMs(const VideoFrame& frame,
+                               int64_t* stream_offset_ms) const;
+
  private:
   rtc::CriticalSection data_cs_;
   VideoCodingModule* const vcm_;
+  Clock* const clock_;
   RtpReceiver* video_receiver_;
   RtpRtcp* video_rtp_rtcp_;
   int voe_channel_id_;