static html5TrackId(html5Track) {
    if (!html5Track['__shaka_id']) {
      html5Track['__shaka_id'] = shaka.util.StreamUtils.nextTrackId_++;
    }
    return html5Track['__shaka_id'];
  }
Generate and return an ID for this track, since the ID field is optional.

@param {TextTrack|AudioTrack|VideoTrack} html5Track
@return {number} The generated ID.