[Offline pages] Extracting and templatizing types for store callbacks

components/offline_pages/offline_page_metadata_store.cc

 // 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.
 
 #include "components/offline_pages/offline_page_metadata_store.h"
 
 namespace offline_pages {
 
-StoreUpdateResult::StoreUpdateResult(StoreState state) : store_state(state) {}
-
-StoreUpdateResult::~StoreUpdateResult() {}
+template class StoreUpdateResult<OfflinePageItem>;
 
 OfflinePageMetadataStore::OfflinePageMetadataStore() {
 }
 
 OfflinePageMetadataStore::~OfflinePageMetadataStore() {
 }
 
 }  // namespace offline_pages