callbackServiceHandler_(message) {
    var signature = message['signature'];
    var result = message['data'];
    var errorResult = message['error'];

    if (signature in this.sigCallbackMap_) {
      var callback = this.sigCallbackMap_[signature].callback;
      var errback = this.sigCallbackMap_[signature].errback;
      delete this.sigCallbackMap_[signature];
      if (errorResult != null) {
        // We have an error.
        if (goog.isFunction(errback)) {
          errback(errorResult);
        } else {
          goog.log.warning(this.logger_, 'No callback to handle error.');
        }
      } else {
        // We have a successful response.
        if (goog.isFunction(callback)) {
          callback(result);
        } else {
          goog.log.warning(this.logger_, 'No callback to handle response.');
        }
      }
    } else {
      goog.log.warning(this.logger_, 'Received signature is invalid');
    }
  }
Receives the results of the peer's service results.
@param {!Object|string} message The results from the remote service
    invocation.
@private