[cros,login] Correctly handle situation when screens are switched very quickly.

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

 // Copyright (c) 2011 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 Display manager for WebUI OOBE and login.
  */
 
 // TODO(xiyuan): Find a better to share those constants.
 const SCREEN_SIGNIN = 'signin';
@@ skipping 133 matching lines @@
         newStep.classList.remove('faded');
       }
 
       // Adjust inner container height based on new step's height.
       $('inner-container').style.height = newStep.offsetHeight + 'px';
 
       if (this.currentStep_ != nextStepIndex &&
           !oldStep.classList.contains('hidden')) {
         oldStep.addEventListener('webkitTransitionEnd', function f(e) {
           oldStep.removeEventListener('webkitTransitionEnd', f);
-          oldStep.classList.add('hidden');
+          if (oldStep.classList.contains('faded') ||
+              oldStep.classList.contains('left') ||
+              oldStep.classList.contains('right')) {
+            oldStep.classList.add('hidden');
+          }
         });
       } else {
         // First screen on OOBE launch.
         newHeader.classList.remove('right');
       }
       this.currentStep_ = nextStepIndex;
       $('oobe').className = nextStepId;
     },
 
     /**
@@ skipping 222 matching lines @@
    */
   DisplayManager.setLabelText = function(labelId, labelText) {
     $(labelId).textContent = labelText;
   };
 
   // Export
   return {
     DisplayManager: DisplayManager
   };
 });