Fixed bug making it impossible to opt-out of auto-enrollment.

chrome/browser/resources/shared/js/cr/ui/dialogs.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.
 
 cr.define('cr.ui.dialogs', function() {
 
   function BaseDialog(parentNode) {
     this.parentNode_ = parentNode;
     this.document_ = parentNode.ownerDocument;
 
@@ skipping 106 matching lines @@
     this.show_(title, onOk, onCancel, onShow);
   };
 
   BaseDialog.prototype.findFocusableElements_ = function(doc) {
     var elements = Array.prototype.filter.call(
         doc.querySelectorAll('*'),
         function(n) { return n.tabIndex >= 0; });
 
     var iframes = doc.querySelectorAll('iframe');
     for (var i = 0; i < iframes.length; i++) {
-      elements = elements.concat(this.findFocusableElements_(
-          iframes[i].contentDocument));
+      // Some iframes have an undefined contentDocument for security reasons,
+      // such as chrome://terms (which is used in the chromeos OOBE screens).
+      var contentDoc = iframes[i].contentDocument;
+      if (contentDoc)
+        elements = elements.concat(this.findFocusableElements_(contentDoc));
     }
     return elements;
   };
 
   BaseDialog.prototype.showWithTitle = function(title, message,
       onOk, onCancel, onShow) {
     this.text_.textContent = message;
     this.show_(title, onOk, onCancel, onShow);
   };
 
@@ skipping 144 matching lines @@
       this.onOk_(this.getValue());
   };
 
   return {
     BaseDialog: BaseDialog,
     AlertDialog: AlertDialog,
     ConfirmDialog: ConfirmDialog,
     PromptDialog: PromptDialog
   };
 });