Add a test helper class for ExtensionRegistry.

chrome/chrome_tests.gypi

 # Copyright 2013 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.
 {
   'includes': [
     'js_unittest_vars.gypi',
   ],
   'targets': [
     {
       'target_name': 'interactive_ui_tests',
@@ skipping 837 matching lines @@
       ],
       'sources': [
         '../apps/app_restore_service_browsertest.cc',
         '../apps/app_window_browsertest.cc',
         '../apps/app_shim/app_shim_host_manager_browsertest_mac.mm',
         '../apps/app_shim/test/app_shim_host_manager_test_api_mac.cc',
         '../apps/app_shim/test/app_shim_host_manager_test_api_mac.h',
         '../apps/load_and_launch_browsertest.cc',
         '../extensions/browser/api/usb/usb_apitest.cc',
         '../extensions/browser/api/usb/usb_manual_apitest.cc',
+        '../extensions/browser/test_extension_registry_observer.h',
+        '../extensions/browser/test_extension_registry_observer.cc',
         '../extensions/renderer/script_context_browsertest.cc',
         # TODO(blundell): Bring up a components_browsertests target and move
         # this test to be in that target. crbug.com/283846
         '../components/autofill/content/renderer/password_form_conversion_utils_browsertest.cc',
         '../components/autofill/content/renderer/test_password_autofill_agent.h',
         '../components/autofill/content/renderer/test_password_autofill_agent.cc',
         '../components/autofill/content/renderer/test_password_generation_agent.h',
         '../components/autofill/content/renderer/test_password_generation_agent.cc',
         # TODO(rockot): Remove this once extensions_browsertests exists.
         '../extensions/browser/api/runtime/runtime_apitest.cc',
@@ skipping 2091 matching lines @@
             ['enable_webrtc==1', {
               'dependencies': [
                 '../third_party/libjingle/libjingle.gyp:libjingle_webrtc'
               ]
             }],
           ],
         }]
     }],
   ],  # 'conditions'
 }