[Extensions] Update extensions UI to observe events and add test

chrome/browser/resources/extensions/extensions.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.
 
 <include src="../../../../ui/webui/resources/js/cr/ui/focus_row.js">
 <include src="../../../../ui/webui/resources/js/cr/ui/focus_grid.js">
 <include src="../uber/uber_utils.js">
 <include src="extension_code.js">
 <include src="extension_commands_overlay.js">
 <include src="extension_error_overlay.js">
@@ skipping 331 matching lines @@
 
     $('load-unpacked').disabled = extensionsData.loadUnpackedDisabled;
     var extensionList = $('extension-settings-list');
     extensionList.updateExtensionsData(
         extensionsData.incognitoAvailable,
         extensionsData.enableAppInfoDialog).then(function() {
       // We can get called many times in short order, thus we need to
       // be careful to remove the 'finished loading' timeout.
       if (this.loadingTimeout_)
         window.clearTimeout(this.loadingTimeout_);
-      document.documentElement.classList.add('loading');
       this.loadingTimeout_ = window.setTimeout(function() {
         document.documentElement.classList.remove('loading');
       }, 0);
 
       var hasExtensions = extensionList.getNumExtensions() != 0;
       $('no-extensions').hidden = hasExtensions;
       $('extension-list-wrapper').hidden = !hasExtensions;
       $('extension-settings-list').updateFocusableElements();
     }.bind(this));
   };
@@ skipping 101 matching lines @@
     document.querySelector('head').appendChild(style);
   };
 
   // Export
   return {
     ExtensionSettings: ExtensionSettings
   };
 });
 
 window.addEventListener('load', function(e) {
+  document.documentElement.classList.add('loading');
   extensions.ExtensionSettings.getInstance().initialize();
 });