requestMediaTailorStream(url, adsParams, backupUrl = '') {
    if (!this.mtAdManager_) {
      throw new shaka.util.Error(
          shaka.util.Error.Severity.RECOVERABLE,
          shaka.util.Error.Category.ADS,
          shaka.util.Error.Code.MT_AD_MANAGER_NOT_INITIALIZED);
    }

    return this.mtAdManager_.streamRequest(url, adsParams, backupUrl);
  }
@param {string} url
@param {Object} adsParams
@param {string=} backupUrl
@return {!Promise<string>}
@override
@export