Use PageManager in About page

chrome/browser/resources/options/alert_overlay.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.
 
 cr.define('options', function() {
   var Page = cr.ui.pageManager.Page;
   var PageManager = cr.ui.pageManager.PageManager;
 
   /**
    * AlertOverlay class
    * Encapsulated handling of a generic alert.

[Dan Beam, 2014/08/06 18:30:54]

nit: @extends {cr.ui.pageManager.Page}

[michaelpg, 2014/08/06 21:58:18]

Done.

    * @class
    */
   function AlertOverlay() {
     Page.call(this, 'alertOverlay', '', 'alertOverlay');
-    // AlertOverlay is special in that it is not tied to one page or overlay.
-    // Set the nesting level arbitrarily high so as to always be recognized as
-    // the top-most visible page.
-    this.nestingLevelOverride = 99;
   }
 
   cr.addSingletonGetter(AlertOverlay);
 
   AlertOverlay.prototype = {
     // Inherit AlertOverlay from Page.
     __proto__: Page.prototype,
 
     /**
      * Whether the page can be shown. Used to make sure the page is only
      * shown via AlertOverlay.Show(), and not via the address bar.
      * @private
      */
     canShow_: false,
 
     /** @override */
     initializePage: function() {
       Page.prototype.initializePage.call(this);
 
+      // AlertOverlay is special in that it is not tied to one page or overlay.
+      this.alwaysOnTop = true;
+
       var self = this;
       $('alertOverlayOk').onclick = function(event) {
         self.handleOK_();
       };
 
       $('alertOverlayCancel').onclick = function(event) {
         self.handleCancel_();
       };
     },
 
@@ skipping 89 matching lines @@
     // Intentionally don't show the URL in the location bar as we don't want
     // people trying to navigate here by hand.
     PageManager.showPageByName('alertOverlay', false);
   };
 
   // Export
   return {
     AlertOverlay: AlertOverlay
   };
 });