webui: remove FocusManager.disableMouseFocusOnButtons().

chrome/browser/resources/extensions/extension_focus_manager.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('extensions', function() {
-  var FocusManager = cr.ui.FocusManager;
-
-  function ExtensionFocusManager() {
-    FocusManager.disableMouseFocusOnButtons();
-  }
+  /**
+   * @constructor
+   * @extends {cr.ui.FocusManager}
+   */
+  function ExtensionFocusManager() {}
 
   cr.addSingletonGetter(ExtensionFocusManager);
 
   ExtensionFocusManager.prototype = {
-    __proto__: FocusManager.prototype,
+    __proto__: cr.ui.FocusManager.prototype,
 
     /** @override */
     getFocusParent: function() {
       var overlay = extensions.ExtensionSettings.getCurrentOverlay();
       return overlay || $('extension-settings');
     },
   };
 
   return {
     ExtensionFocusManager: ExtensionFocusManager,
   };
 });