Create skeleton for the Prefetching store and initial pipeline step.

components/offline_pages/content/prefetch_service_factory.cc

 // 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.
 
 #include "components/offline_pages/content/prefetch_service_factory.h"
 
+#include <memory>
+
 #include "base/memory/singleton.h"
 #include "components/keyed_service/content/browser_context_dependency_manager.h"
 #include "components/offline_pages/core/prefetch/prefetch_service_impl.h"
+#include "components/offline_pages/core/prefetch/prefetch_store.h"
 #include "content/public/browser/browser_context.h"
 
 namespace offline_pages {
 
 PrefetchServiceFactory::PrefetchServiceFactory()
     : BrowserContextKeyedServiceFactory(
           "OfflinePagePrefetchService",
           BrowserContextDependencyManager::GetInstance()) {}
 
 // static
 PrefetchServiceFactory* PrefetchServiceFactory::GetInstance() {
   return base::Singleton<PrefetchServiceFactory>::get();
 }
 
 // static
 PrefetchService* PrefetchServiceFactory::GetForBrowserContext(
     content::BrowserContext* context) {
   return static_cast<PrefetchService*>(
       GetInstance()->GetServiceForBrowserContext(context, true));
 }
 
 KeyedService* PrefetchServiceFactory::BuildServiceInstanceFor(
     content::BrowserContext* context) const {
-  return new PrefetchServiceImpl();
+  // TODO(carlosk): instantiate actual implementation of the store once it
+  // exists.
+  std::unique_ptr<PrefetchStore> prefetch_store;

[Dmitry Titov, 2017/05/18 21:24:02]

Does this create an instance of abstract class? Do you want to add a
PrefetchStoreImpl with trivial implementaiton and add it here?

[carlosk, 2017/05/24 20:57:26]

This stores a null pointer. And I like the idea of getting clean null crashes
while this is not implemented. Adding the base implementation is part of the
followup step.

+  return new PrefetchServiceImpl(std::move(prefetch_store));
 }
 
 }  // namespace offline_pages