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

chrome/browser/resources/chromeos/login/screen_gaia_signin.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 Oobe signin screen implementation.
  */
 
 cr.define('login', function() {
   // Gaia loading time after which error message must be displayed and
@@ skipping 260 matching lines @@
       $('createLocallyManagedUser').hidden = !data.createLocallyManagedUser;
       // Allow cancellation of screen only when user pods can be displayed.
       this.cancelAllowed_ = data.isShowUsers && $('pod-row').pods.length;
       $('login-header-bar').allowCancel = this.cancelAllowed_;
 
       // Sign-in right panel is hidden if all of its items are hidden.
       var noRightPanel = $('gaia-signin-reason').hidden &&
                          $('createAccount').hidden &&
                          $('guestSignin').hidden &&
                          $('createLocallyManagedUser').hidden;
-      this.classList[noRightPanel ? 'add' : 'remove']('no-right-panel');
+      this.classList.toggle('no-right-panel', noRightPanel);
       if (Oobe.getInstance().currentScreen === this)
         Oobe.getInstance().updateScreenSize(this);
     },
 
     /**
      * Checks if message comes from the loaded authentication extension.
      * @param {object} e Payload of the received HTML5 message.
      * @type {boolean}
      */
     isAuthExtMessage_: function(e) {
@@ skipping 197 matching lines @@
     // Check current network state if currentScreen is a Gaia signin.
     var currentScreen = Oobe.getInstance().currentScreen;
     if (currentScreen.id == SCREEN_GAIA_SIGNIN)
       chrome.send('showGaiaFrameError', [error]);
   };
 
   return {
     GaiaSigninScreen: GaiaSigninScreen
   };
 });