static setVerticalWritingMode_(cue, value) {
    const Cue = shaka.text.Cue;
    if (value == 'lr') {
      cue.writingMode = Cue.writingMode.VERTICAL_LEFT_TO_RIGHT;
    } else {
      cue.writingMode = Cue.writingMode.VERTICAL_RIGHT_TO_LEFT;
    }
  }
@param {!shaka.text.Cue} cue
@param {string} value
@private