Use PageManager in About page

chrome/browser/resources/help/help_focus_manager.js

-// Copyright 2013 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.
-
-// Helper class for help page, that controls focus transition between
-// elements on the help page and overlays.
-cr.define('help', function() {
-  function HelpFocusManager() {
-  }
-
-  cr.addSingletonGetter(HelpFocusManager);
-
-  HelpFocusManager.prototype = {
-    __proto__: cr.ui.FocusManager.prototype,
-
-    getFocusParent: function() {
-      var page = help.HelpPage.getTopmostVisiblePage();
-      if (!page)
-        return null;
-      return page.pageDiv;
-    },
-  };
-
-  return {
-    HelpFocusManager: HelpFocusManager,
-  };
-});