static createLineBreakCue(startTime, endTime) {
    const cue = new shaka.text.Cue(startTime, endTime, /* payload= */ '');
    cue.lineBreak = true;
    return cue;
  }
Returns a cue that corresponds to a linebreak.
@param {number} startTime
@param {number} endTime
@return {!shaka.text.Cue}