send(serviceName, payload, callback, errback) {
    var signature = this.getNextSignature_();
    this.sigCallbackMap_[signature] = {
      'callback': callback,
      'errback': errback
    };

    var message = {};
    message['signature'] = signature;
    message['data'] = payload;

    this.publicChannel_.send(serviceName, message);
  }
Sends a message over the channel.
@param {string} serviceName The name of the service this message should be
    delivered to.
@param {string|!Object} payload The value of the message. If this is an
    Object, it is serialized to a string before sending if necessary.
@param {function(?Object)} callback The callback invoked with
    a successful result of the service call.
@param {function(?Object)} errback The callback invoked with
    an erroneous result of the service call.