async makeAdRequest_(url, context) {
    const type = shaka.net.NetworkingEngine.RequestType.ADS;
    const request = shaka.net.NetworkingEngine.makeRequest(
        [url],
        shaka.net.NetworkingEngine.defaultRetryParameters());
    const op = this.basePlayer_.getNetworkingEngine()
        .request(type, request, context);
    const response = await op.promise;
    return response.data;
  }
@param {string} url
@param {shaka.extern.RequestContext=} context
@return {!Promise<BufferSource>}
@private