Add onChanged events to the extension settings API, both from sync and between

chrome/chrome_browser.gypi

 # Copyright (c) 2011 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.
 
 {
   'targets': [
     {
       'target_name': 'browser',
       'type': 'static_library',
       'dependencies': [
@@ skipping 1074 matching lines @@
         'browser/extensions/extension_proxy_api_constants.cc',
         'browser/extensions/extension_proxy_api_constants.h',
         'browser/extensions/extension_proxy_api_helpers.cc',
         'browser/extensions/extension_proxy_api_helpers.h',
         'browser/extensions/extension_rlz_module.cc',
         'browser/extensions/extension_rlz_module.h',
         'browser/extensions/extension_save_page_api.cc',
         'browser/extensions/extension_save_page_api.h',
         'browser/extensions/extension_setting_sync_data.cc',
         'browser/extensions/extension_setting_sync_data.h',
+        'browser/extensions/extension_settings_api.cc',
+        'browser/extensions/extension_settings_api.h',
         'browser/extensions/extension_settings_backend.cc',
         'browser/extensions/extension_settings_backend.h',
         'browser/extensions/extension_settings_frontend.cc',
         'browser/extensions/extension_settings_frontend.h',
-        'browser/extensions/extension_settings_api.cc',
-        'browser/extensions/extension_settings_api.h',
+        'browser/extensions/extension_settings_observer.cc',
+        'browser/extensions/extension_settings_observer.h',
         'browser/extensions/extension_settings_storage.cc',
         'browser/extensions/extension_settings_storage.h',
         'browser/extensions/extension_settings_storage_cache.cc',
         'browser/extensions/extension_settings_storage_cache.h',
         'browser/extensions/extension_settings_sync_util.cc',
         'browser/extensions/extension_settings_sync_util.h',
         'browser/extensions/extension_settings_storage_quota_enforcer.cc',
         'browser/extensions/extension_settings_storage_quota_enforcer.h',
         'browser/extensions/extension_settings_leveldb_storage.cc',
         'browser/extensions/extension_settings_leveldb_storage.h',
@@ skipping 3969 matching lines @@
       'type': 'static_library',
       'sources': [ 'browser/history/in_memory_url_index_cache.proto' ],
       'variables': {
         'proto_in_dir': 'browser/history',
         'proto_out_dir': 'chrome/browser/history',
       },
       'includes': [ '../build/protoc.gypi' ]
     },
   ],
 }