RTCRtpStreamStats: trackId property
  The RTCRtpStreamStats dictionary's
  trackId property is a string which uniquely identifies
  the RTCMediaStreamTrackStats object which contains the track statistics
  for the MediaStreamTrack for which statistics are provided in this
  object.
Value
  A string which uniquely identifies the
  RTCMediaStreamTrackStats object that provides statistics for the track
  for which statistics are being collected by this RTCStatsReport.
    Note: This value is not the same as the value of
    MediaStreamTrack.id.
  
Specifications
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No specification data found for api.RTCRtpStreamStats.trackId.
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