[Offline] Adding archiver interface

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.
 
 # GYP: //components/offline_pages.gypi:offline_pages
 static_library("offline_pages") {
   sources = [
+    "offline_page_archiver.h",
     "offline_page_item.cc",
     "offline_page_item.h",
     "offline_page_metadata_store.cc",
     "offline_page_metadata_store.h",
     "offline_page_model.cc",
     "offline_page_model.h",
   ]
 
   deps = [
     "//base",
     "//components/keyed_service/core",
     "//net",
     "//url",
   ]
 }
 
 source_set("unit_tests") {
   testonly = true
   sources = [
     "offline_page_model_unittest.cc",
   ]
 
   deps = [
     ":offline_pages",
     "//testing/gtest",
   ]
 }