MD User Manager: Refactors existing dialogs in User Manager into <user-manager-dialog>

chrome/test/data/webui/md_user_manager/control_bar_tests.js

 // Copyright 2016 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('user_manager.control_bar_tests', function() {
   /** @return {!ControlBarElement} */
   function createElement() {
     var controlBarElement = document.createElement('control-bar');
     document.body.appendChild(controlBarElement);
     return controlBarElement;
@@ skipping 77 matching lines @@
 
       test('Cannot create profile', function() {
         // Simulate clicking 'Create Profile'.
         MockInteractions.tap(controlBarElement.$.addUser);
 
         return browserProxy.whenCalled('areAllProfilesLocked').then(function() {
           // Make sure DOM is up to date.
           Polymer.dom.flush();
 
           // The dialog is visible.
-          assertLT(0, errorDialogElement.$$('#backdrop').offsetHeight);
+          assertTrue(errorDialogElement.$.dialog.opened);
         });
       });
 
       test('Cannot launch guest profile', function() {
         // Simulate clicking 'Browse as guest'.
         MockInteractions.tap(controlBarElement.$.launchGuest);
 
         return browserProxy.whenCalled('areAllProfilesLocked').then(function() {
           // Make sure DOM is up to date.
           Polymer.dom.flush();
 
           // The error dialog is visible.
-          assertLT(0, errorDialogElement.$$('#backdrop').offsetHeight);
+          assertTrue(errorDialogElement.$.dialog.opened);
         });
       });
     });
   }
 
   return {
     registerTests: registerTests,
   };
 });