Add about flag for offline page experiment

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',
@@ skipping 14 matching lines @@
         'offline_pages/offline_page_feature.cc',
         'offline_pages/offline_page_feature.h',
         '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',
         'offline_pages/offline_page_metadata_store_impl.cc',
         'offline_pages/offline_page_metadata_store_impl.h',
+        'offline_pages/offline_page_switches.cc',
+        'offline_pages/offline_page_switches.h',
       ],
     },
     {
       # Protobuf compiler / generator for the offline page item protocol buffer.
       # GN version: //components/offline_pages/proto
       'target_name': 'offline_pages_proto',
       'type': 'static_library',
       'sources': [ 'offline_pages/proto/offline_pages.proto', ],
       'variables': {
         'proto_in_dir': 'offline_pages/proto',
@@ skipping 11 matching lines @@
           'type': 'none',
           'variables': {
             'source_file': 'offline_pages/offline_page_model.h',
           },
           'includes': [ '../build/android/java_cpp_enum.gypi' ],
         },
       ],
     }],
   ],
 }