static convertTTMLrgbaToHTMLrgba_(color) {
    const rgba = color.match(/rgba\(([^)]+)\)/);
    if (rgba) {
      const values = rgba[1].split(',');
      if (values.length == 4) {
        values[3] = String(Number(values[3]) / 255);
        return 'rgba(' + values.join(',') + ')';
      }
    }
    return color;
  }
Ensures any TTML RGBA's alpha range of 0-255 is converted to 0-1.
@param {string} color
@return {string}
@private