trackId property is a string which uniquely identifies
RTCMediaStreamTrackStats object which contains the track statistics
MediaStreamTrack for which statistics are provided in this
var trackID = RTCRtpStreamStats.trackId;
Note: This value is not the same as the value of
|Identifiers for WebRTC's Statistics API
The definition of 'RTCRtpStreamStats.trackId' in that specification.
|Candidate Recommendation||Initial definition.|