MD WebUI: change how initial focus works in <dialog>s now that they know about shadow DOM

chrome/test/data/webui/cr_elements/cr_dialog_test.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.
 
 suite('cr-dialog', function() {
   setup(function() {
     PolymerTest.clearBody();
   });
 
   test('focuses title on show', function() {
     document.body.innerHTML = `
       <dialog is="cr-dialog">
         <div class="title">title</div>
         <div class="body"><button>button</button></div>
       </dialog>`;
 
-    assertFalse(document.activeElement.matches('div.title'));
-    assertFalse(document.activeElement.matches('button'));
+    var dialog = document.body.querySelector('dialog');
+    var button = document.body.querySelector('button');
 
-    document.body.querySelector('dialog').showModal();
+    assertNotEquals(dialog, document.activeElement);
+    assertNotEquals(button, document.activeElement);
 
-    expectTrue(document.activeElement.matches('div.title'));
-    expectFalse(document.activeElement.matches('button'));
+    dialog.showModal();
+
+    expectEquals(dialog, document.activeElement);
+    expectNotEquals(button, document.activeElement);
   });
 
   test('focuses [autofocus] instead of title when present', function() {
     document.body.innerHTML = `
       <dialog is="cr-dialog">
         <div class="title">title</div>
         <div class="body"><button autofocus>button</button></div>
       </dialog>`;
 
-    assertFalse(document.activeElement.matches('button'));
-    assertFalse(document.activeElement.matches('div.title'));
+    var dialog = document.body.querySelector('dialog');
+    var button = document.body.querySelector('button');
 
-    document.body.querySelector('dialog').showModal();
+    assertNotEquals(dialog, document.activeElement);
+    assertNotEquals(button, document.activeElement);
 
-    expectTrue(document.activeElement.matches('button'));
-    expectFalse(document.activeElement.matches('div.title'));
+    dialog.showModal();
+
+    expectNotEquals(dialog, document.activeElement);
+    expectEquals(button, document.activeElement);
   });
 });