Add about flag for offline page experiment

components/offline_pages/BUILD.gn

 # 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.
 
 if (is_android) {
   import("//build/config/android/rules.gni")
 }
 
 # GYP: //components/offline_pages.gypi:offline_pages
 static_library("offline_pages") {
   sources = [
     "offline_page_archiver.h",
     "offline_page_feature.cc",
     "offline_page_feature.h",
     "offline_page_item.cc",
     "offline_page_item.h",
     "offline_page_metadata_store.cc",
     "offline_page_metadata_store.h",
     "offline_page_metadata_store_impl.cc",
     "offline_page_metadata_store_impl.h",
     "offline_page_model.cc",
     "offline_page_model.h",
+    "offline_page_switches.cc",
+    "offline_page_switches.h",
   ]
 
   deps = [
     "//base",
     "//components/keyed_service/core",
     "//components/leveldb_proto",
     "//components/offline_pages/proto:offline_pages_proto",
     "//net",
     "//third_party/leveldatabase",
     "//url",
@@ skipping 19 matching lines @@
     sources = [
       "offline_page_model.h",
     ]
     outputs = [
       "org/chromium/components/offline_pages/DeletePageResult.java",
       "org/chromium/components/offline_pages/LoadResult.java",
       "org/chromium/components/offline_pages/SavePageResult.java",
     ]
   }
 }