Use classList#toggle instead of classList[whatever ? 'add' : 'remove'].

chrome/browser/resources/chromeos/login/screen_error_message.js

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 /**
  * @fileoverview Offline message screen implementation.
  */
 
 cr.define('login', function() {
   // Link which starts guest session for captive portal fixing.
@@ skipping 178 matching lines @@
     onContentChange_: function() {
       if (Oobe.getInstance().currentScreen === this)
         Oobe.getInstance().updateScreenSize(this);
     },
 
     /**
      * Prepares error screen to show guest signin link.
      * @private
      */
     allowGuestSignin_: function(allowed) {
-      this.classList[allowed ? 'add' : 'remove']('allow-guest-signin');
+      this.classList.toggle('allow-guest-signin', allowed);
       this.onContentChange_();
     },
 
     /**
      * Prepares error screen to show offline login link.
      * @private
      */
     allowOfflineLogin_: function(allowed) {
-      this.classList[allowed ? 'add' : 'remove']('allow-offline-login');
+      this.classList.toggle('allow-offline-login', allowed);
       this.onContentChange_();
     },
   };
 
   /**
     * Sets current UI state of the screen.
     * @param {number} ui_state New UI state of the screen.
     * @private
     */
   ErrorMessageScreen.setUIState = function(ui_state) {
@@ skipping 39 matching lines @@
    * via template.
    */
   ErrorMessageScreen.updateLocalizedContent = function() {
     $('error-message').updateLocalizedContent_();
   };
 
   return {
     ErrorMessageScreen: ErrorMessageScreen
   };
 });