Add base persistence interfaces for Prefetching Offline Pages.

components/offline_pages/core/prefetch/BUILD.gn

 # Copyright 2017 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")
 }
 
 static_library("prefetch") {
   sources = [
+    "prefetch_item.cc",
+    "prefetch_item.h",
     "prefetch_request_fetcher.cc",
     "prefetch_request_fetcher.h",
     "prefetch_service.h",
     "prefetch_service_impl.cc",
     "prefetch_service_impl.h",
+    "prefetch_types.h",
   ]
 
   deps = [
     "//base",
     "//components/keyed_service/core",
     "//components/offline_pages/core",
     "//net:net",
     "//url",
   ]
 }
 
 source_set("unit_tests") {
   testonly = true
   sources = [
     "prefetch_request_fetcher_unittest.cc",
     "prefetch_service_impl_unittest.cc",
   ]
 
   deps = [
     ":prefetch",
     "//components/offline_pages/core",
     "//net:test_support",
     "//testing/gmock",
     "//testing/gtest",
     "//url",
   ]
 }