sendMessage_(message, bus, senderId) {
    // Cuts log spam when debugging the receiver UI in Chrome.
    if (!this.isConnected_) {
      return;
    }

    const serialized = shaka.cast.CastUtils.serialize(message);
    if (senderId) {
      bus.getCastChannel(senderId).send(serialized);
    } else {
      bus.broadcast(serialized);
    }
  }
Since this method is in the compiled library, make sure all messages passed
in here were created with quoted property names.
@param {!Object} message
@param {cast.receiver.CastMessageBus} bus
@param {string=} senderId
@private