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

During the OOBE flow there's an iframe pointing to "chrome://terms", whose
contentDocument is undefined.

BUG=chromium-os:27045
TEST=Steps in the bug now work as expected.

Review URL: https://chromiumcodereview.appspot.com/9594024

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@124965 0039d316-1c4b-4281-b951-d872f2087c98

chrome/browser/resources/shared/js/cr/ui/dialogs.js

@@ -124,8 +124,11 @@ cr.define('cr.ui.dialogs', function() {
 
     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;
   };