showDisconnectDialog() {
    if (!this.isCasting_) {
      return;
    }

    chrome.cast.requestSession(
        (session) => this.onSessionInitiated_(session),
        (error) => this.onConnectionError_(error));
  }
Shows user a cast dialog where they can choose to stop
casting.  Relies on Chrome to perform disconnect if they do.
Doesn't do anything if not connected.