static getRegionById_(regions, id) {
    const regionsWithId = regions.filter((region) => {
      return region.id == id;
    });
    if (!regionsWithId.length) {
      shaka.log.warning('VTT parser could not find a region with id: ',
          id,
          ' The region will be ignored.');
      return null;
    }
    goog.asserts.assert(regionsWithId.length == 1,
        'VTTRegion ids should be unique!');

    return regionsWithId[0];
  }
@param {!Array<!shaka.text.CueRegion>} regions
@param {string} id
@return {?shaka.text.CueRegion}
@private