[Offline] Creates metadata store interface for offline pages

components/offline_pages.gypi

 # Copyright 2015 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': [
     {
       # GN: //components/offline_pages:offline_pages
       'target_name': 'offline_pages',
       'type': 'static_library',
       'include_dirs': [
         '..',
       ],
       'dependencies': [
         '../base/base.gyp:base',
         '../net/net.gyp:net',
         '../url/url.gyp:url_lib',
         'keyed_service_core',
       ],
       'sources': [
         'offline_pages/offline_page_item.cc',
         'offline_pages/offline_page_item.h',
         'offline_pages/offline_page_model.cc',
         'offline_pages/offline_page_model.h',
+        'offline_pages/offline_page_metadata_store.cc',
+        'offline_pages/offline_page_metadata_store.h',
       ],
     },
   ],
 }