static convertLengthValue_(lengthValue, cue, videoContainer,
      scaleFactor = 1) {
    const lengthValueInfo =
        shaka.text.UITextDisplayer.getLengthValueInfo_(lengthValue);

    if (!lengthValueInfo) {
      return lengthValue;
    }

    const {unit, value} = lengthValueInfo;

    const realValue = value * scaleFactor;

    switch (unit) {
      case '%':
        return shaka.text.UITextDisplayer.getAbsoluteLengthInPixels_(
            realValue / 100, cue, videoContainer);
      case 'c':
        return shaka.text.UITextDisplayer.getAbsoluteLengthInPixels_(
            realValue, cue, videoContainer);
      default:
        return realValue + unit;
    }
  }
Converts length value to an absolute value in pixels.
If lengthValue is already an absolute value it will not
be modified. Relative lengthValue will be converted to an
absolute value in pixels based on Computed Cell Size

@param {string} lengthValue
@param {!shaka.text.Cue} cue
@param {HTMLElement} videoContainer
@param {number=} scaleFactor
@return {string}
@private