Enable sync for the settings from the Extension Settings API.

chrome/browser/sync/protocol/sync_proto.gyp

 # Copyright (c) 2010 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': [
     {
       'target_name': 'sync_proto',
       'type': 'static_library',
       'sources': [
         'sync.proto',
         'encryption.proto',
         'app_specifics.proto',
         'autofill_specifics.proto',
         'bookmark_specifics.proto',
+        'extension_setting_specifics.proto',
         'extension_specifics.proto',
         'nigori_specifics.proto',
         'password_specifics.proto',
         'preference_specifics.proto',
         'search_engine_specifics.proto',
         'session_specifics.proto',
         'test.proto',
         'theme_specifics.proto',
         'typed_url_specifics.proto',
       ],
       'variables': {
         'proto_out_dir': 'chrome/browser/sync/protocol',
       },
       'includes': ['../../../../build/protoc.gypi'],
     },
   ],
 }