widgetUrlForMode_(baseUrl, mode = undefined) {
    if (mode) {
      const key = this.getQueryParameterForWidgetMode();
      return utils.setParam(baseUrl, key, mode);
    } else {
      return baseUrl;
    }
  }
@param {string} baseUrl The base URL of the widget.
@param {?Config.WidgetMode=} mode The mode for the widget.
@return {string} The URL of the widget for a specific mode.
@private