sendResponse_(resultMessage) {
    if (this.privateChannel_) {
      // The callback invoked above may have disposed the channel so check if it
      // exists.
      this.privateChannel_.send(
          e2e.messaging.AsyncRespondingChannel.CALLBACK_SERVICE_,
          resultMessage);
    }
  }
Sends response to a certain message to the other peer.
@param {!Object} resultMessage The result message object to send back.
@private