Use PageManager in About page

chrome/browser/resources/options/alert_overlay.js

Index: chrome/browser/resources/options/alert_overlay.js
diff --git a/chrome/browser/resources/options/alert_overlay.js b/chrome/browser/resources/options/alert_overlay.js
index 09b95bf7db87915dd65e822107702c7e275c696d..cd7aaf36a093207d6206c41a33652162cd8f892d 100644
--- a/chrome/browser/resources/options/alert_overlay.js
+++ b/chrome/browser/resources/options/alert_overlay.js
@@ -9,14 +9,11 @@ cr.define('options', function() {
   /**
    * AlertOverlay class
    * Encapsulated handling of a generic alert.
-   * @class
+   * @constructor
+   * @extends {cr.ui.pageManager.Page}
    */
   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);
@@ -36,6 +33,9 @@ cr.define('options', function() {
     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_();