First try at a user management screen for the desktop

chrome/browser/resources/chromeos/login/screen_account_picker.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 Account picker screen implementation.
  */
 
 login.createScreen('AccountPickerScreen', 'account-picker', function() {
   /**
@@ skipping 56 matching lines @@
       chrome.send('loginUIStateChanged', ['account-picker', true]);
       $('login-header-bar').signinUIState = SIGNIN_UI_STATE.ACCOUNT_PICKER;
       chrome.send('hideCaptivePortal');
       var podRow = $('pod-row');
       podRow.handleBeforeShow();
 
       // If this is showing for the lock screen display the sign out button,
       // hide the add user button and activate the locked user's pod.
       var lockedPod = podRow.lockedPod;
       $('add-user-header-bar-item').hidden = !!lockedPod;
-      $('sign-out-user-item').hidden = !lockedPod;
+      var signOutUserItem = $('sign-out-user-item');
+      if (signOutUserItem)
+        signOutUserItem.hidden = !lockedPod;
       // In case of the preselected pod onShow will be called once pod
       // receives focus.
       if (!podRow.preselectedPod)
         this.onShow();
     },
 
     /**
      * Event handler invoked when the page is shown and ready.
      */
     onShow: function() {
@@ skipping 86 matching lines @@
      * Enforces focus on user pod of locked user.
      */
     forceLockedUserPodFocus: function() {
       var row = $('pod-row');
       if (row.lockedPod)
         row.focusPod(row.lockedPod, true);
     }
   };
 });