_setRatingClass(element, score, scoreDisplayMode) {
    const rating = ReportUtils.calculateRating(score, scoreDisplayMode);
    element.classList.add(`lh-audit--${scoreDisplayMode.toLowerCase()}`);
    if (scoreDisplayMode !== 'informative') {
      element.classList.add(`lh-audit--${rating}`);
    }
    return element;
  }
@param {Element} element DOM node to populate with values.
@param {number|null} score
@param {string} scoreDisplayMode
@return {!Element}