Add ExtensionsApiClient interface, use it in the storage API SettingsFrontend

extensions/extensions.gyp

 # 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': {
     'chromium_code': 1,
   },
   'targets': [
     {
@@ skipping 152 matching lines @@
         # Needed to access generated API headers.
         '<(SHARED_INTERMEDIATE_DIR)',
         # Needed for grit.
         '<(SHARED_INTERMEDIATE_DIR)/chrome',
       ],
       'sources': [
         'browser/admin_policy.cc',
         'browser/admin_policy.h',
         'browser/api/async_api_function.cc',
         'browser/api/async_api_function.h',
+        'browser/api/extensions_api_client.cc',
+        'browser/api/extensions_api_client.h',
+        'browser/api/storage/settings_namespace.cc',
+        'browser/api/storage/settings_namespace.h',
         'browser/api_activity_monitor.h',
         'browser/app_sorting.h',
         'browser/blacklist_state.h',
         'browser/error_map.cc',
         'browser/error_map.h',
         'browser/event_listener_map.cc',
         'browser/event_listener_map.h',
         'browser/event_router.cc',
         'browser/event_router.h',
         'browser/extension_error.cc',
@@ skipping 92 matching lines @@
         'common/test_util.cc',
         'common/test_util.h',
         'common/value_builder.cc',
         'common/value_builder.h',
       ],
       # Disable c4267 warnings until we fix size_t to int truncations.
       'msvs_disabled_warnings': [ 4267, ],
     },
   ]
 }