getGeneratedTextTrack_() {
    goog.asserts.assert(this.video_.textTracks,
        'TextTracks should be valid.');
    return Array.from(this.video_.textTracks)
        .find((t) => t.label == shaka.Player.TextTrackLabel);
  }
Get the one text track generated by the SimpleTextDisplayer.

@return {?TextTrack}
@private