[webrtc-stats] Stats to keep track of sync between audio and video

henbos has just created a new issue for 
https://github.com/w3c/webrtc-stats:

== Stats to keep track of sync between audio and video ==
It would be useful to be able to detect the audio and video of a 
stream becoming out-of-sync. There should be a current playout 
timestamp for remote audio and video tracks. They should be 
monotonically increasing and comparable between tracks from the same 
remote source.

Shall we add `DOMHighResTimeStamp playoutTimestamp` to 
`RTCMediaStreamTrackStats`?

Please view or discuss this issue at 
https://github.com/w3c/webrtc-stats/issues/158 using your GitHub 
account