[Extensions Page MD] Stub out a materialized extensions page

chrome/browser/resources/md_extensions/manager.js

 // Copyright 2015 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.
 
-Polymer({
-  is: 'gaia-header',
+cr.define('extensions', function() {
+  var Manager = Polymer({
+    is: 'extensions-manager'
+  });
 
-  properties: {
-    email: String
-  }
+  return {Manager: Manager};
 });