Fix animation when toggling Developer Mode on Extensions page.

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="../shared/js/cr/ui/drag_wrapper.js"></include>
 <include src="../uber/uber_utils.js"></include>
 <include src="extension_commands_overlay.js"></include>
 <include src="extension_focus_manager.js"></include>
 <include src="extension_list.js"></include>
 <include src="pack_extension_overlay.js"></include>
@@ skipping 219 matching lines @@
       $('toggle-dev-on').disabled = true;
     } else {
       pageDiv.classList.remove('showing-banner');
       pageDiv.classList.remove('managed-mode');
       $('toggle-dev-on').disabled = false;
     }
 
     if (extensionsData.developerMode && !extensionsData.managedMode) {
       pageDiv.classList.add('dev-mode');
       $('toggle-dev-on').checked = true;
+      $('dev-controls').hidden = false;
     } else {
       pageDiv.classList.remove('dev-mode');
       $('toggle-dev-on').checked = false;
     }
 
     $('load-unpacked').disabled = extensionsData.loadUnpackedDisabled;
 
     ExtensionsList.prototype.data_ = extensionsData;
     var extensionList = $('extension-settings-list');
     ExtensionsList.decorate(extensionList);
@@ skipping 81 matching lines @@
   return {
     ExtensionSettings: ExtensionSettings
   };
 });
 
 var ExtensionSettings = extensions.ExtensionSettings;
 
 window.addEventListener('load', function(e) {
   ExtensionSettings.getInstance().initialize();
 });