Use element.createShadowRoot since WebKitShadowRoot is deprecated.

chrome/renderer/resources/extensions/inject_app_titlebar.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.
 
 exports.didCreateDocumentElement = function() {
-  var root = new WebKitShadowRoot(document.documentElement);
+  var root = document.documentElement.webkitCreateShadowRoot();
   root.appendChild(document.createElement('style')).innerText =
       // TODO(jeremya): switch this to use automatic inlining once grit
       // supports inlining into JS. See http://crbug.com/146319.
       "x-titlebar { height: 24px; width: 100%; " +
         "position: fixed; left: 0; top: 0; }\n" +
       "div { margin-top: 24px; position: absolute; top: 0px; width: 100%; " +
         "-webkit-widget-region: region(control rectangle); }\n" +
       ":-webkit-full-screen * { display: none; }\n" +
       ":-webkit-full-screen-document * { display: none; }\n" +
       "div:-webkit-full-screen, div:-webkit-full-screen-document { " +
@@ skipping 23 matching lines @@
       "oG2AFd1AfERUB8CM11WOXQXXAGSROyITDNMGkTGAPdAHSFIENAAOQqGEBxHbYwQDcE2ScY" +
       "XsMViNgMwRYuOGOBIgMo8gLFgUi1aCQ7IZGcNaieF0h2AQCMABwRdsuhtQAAAABJRU5Erk" +
       "Jggg==); }\n"
   var titlebar = root.appendChild(document.createElement('x-titlebar'));
   var closeButton = titlebar.appendChild(document.createElement('button'));
   closeButton.className = 'close'
   closeButton.addEventListener('click', function() { window.close(); });
   var container = root.appendChild(document.createElement('div'));
   container.appendChild(document.createElement('content'));
 }