Add hook for extension to check sync status.

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.
 {
   'variables' : {
     'chromedriver_unittest_sources': [
       'test/chromedriver/capabilities_unittest.cc',
       'test/chromedriver/chrome/chrome_finder_unittest.cc',
       'test/chromedriver/chrome/console_logger_unittest.cc',
       'test/chromedriver/chrome/device_manager_unittest.cc',
@@ skipping 1129 matching lines @@
         'browser/extensions/api/media_galleries_private/media_galleries_watch_apitest.cc',
         'browser/extensions/api/messaging/native_messaging_apitest.cc',
         'browser/extensions/api/metrics_private/metrics_apitest.cc',
         'browser/extensions/api/module/module_apitest.cc',
         'browser/extensions/api/music_manager_private/music_manager_private_browsertest.cc',
         'browser/extensions/api/notifications/notifications_apitest.cc',
         'browser/extensions/api/omnibox/omnibox_api_browsertest.cc',
         'browser/extensions/api/page_capture/page_capture_apitest.cc',
         'browser/extensions/api/permissions/permissions_apitest.cc',
         'browser/extensions/api/preference/preference_apitest.cc',
+        'browser/extensions/api/preferences_private/preferences_private_apitest.cc',
         'browser/extensions/api/processes/processes_apitest.cc',
         'browser/extensions/api/proxy/proxy_apitest.cc',
         'browser/extensions/api/push_messaging/push_messaging_apitest.cc',
         'browser/extensions/api/push_messaging/push_messaging_canary_test.cc',
         'browser/extensions/api/push_messaging/sync_setup_helper.cc',
         'browser/extensions/api/runtime/runtime_apitest.cc',
         'browser/extensions/api/serial/serial_apitest.cc',
         'browser/extensions/api/sessions/sessions_apitest.cc',
         'browser/extensions/api/settings_overrides/settings_overrides_browsertest.cc',
         'browser/extensions/api/socket/socket_apitest.cc',
@@ skipping 1928 matching lines @@
             'utility',
           ],
           'sources': [
             'tools/service_discovery_sniffer/service_discovery_sniffer.h',
             'tools/service_discovery_sniffer/service_discovery_sniffer.cc',
           ],
         }]
     }],
   ],  # 'conditions'
 }