getTosUrl() {
    const tosUrl = this.config_.get('tosUrl') || null;
    const privacyPolicyUrl = this.config_.get('privacyPolicyUrl') || null;
    if (tosUrl && !privacyPolicyUrl) {
      log.warning(
          'Privacy Policy URL is missing, the link will not be displayed.');
    }
    if (tosUrl && privacyPolicyUrl) {
      if (typeof tosUrl === 'function') {
        return /** @type {function()} */ (tosUrl);
      } else if (typeof tosUrl === 'string') {
        return () => {
          util.open(
              /** @type {string} */ (tosUrl),
              util.isCordovaInAppBrowserInstalled() ?
              '_system' : '_blank');
        };
      }
    }
    return null;
  }
@return {?function()} The ToS callback for the site. If URL is provided,
    wraps the URL with a callback function.