async onEncryptedEvent_(event) {
    /**
     * MediaKeys should be added when receiving an encrypted event. Setting
     * mediaKeys before could result into encrypted event not being fired on
     * some browsers
     */
    await this.attachMediaKeys_();

    this.newInitData(
        event.initDataType,
        shaka.util.BufferUtils.toUint8(event.initData));
  }
Processes encrypted event and start licence challenging
@return {!Promise}
@private