[Offline Pages] Set up the initial prefetching service.

chrome/browser/android/offline_pages/prefetch_service_factory.h

+// 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.
+
+#ifndef CHROME_BROWSER_ANDROID_OFFLINE_PAGES_PREFETCH_SERVICE_FACTORY_H_
+#define CHROME_BROWSER_ANDROID_OFFLINE_PAGES_PREFETCH_SERVICE_FACTORY_H_
+
+#include "base/macros.h"
+#include "components/keyed_service/content/browser_context_keyed_service_factory.h"
+
+namespace base {
+template <typename T>
+struct DefaultSingletonTraits;
+}  // namespace base
+
+namespace offline_pages {
+
+class PrefetchService;
+
+// A factory to create one unique PrefetchService. Prefetching Offline Pages is

[jianli, 2017/04/17 21:52:58]

nit: "one unique PrefetchService" => "PrefetchService instances per
BrowserContext"

[dewittj, 2017/04/17 22:22:18]

Done.

+// not supported in incognito, so this class uses default implementation of
+// |GetBrowserContextToUse|.
+class PrefetchServiceFactory : public BrowserContextKeyedServiceFactory {
+ public:
+  static PrefetchServiceFactory* GetInstance();
+  static PrefetchService* GetForBrowserContext(
+      content::BrowserContext* context);
+
+ private:
+  friend struct base::DefaultSingletonTraits<PrefetchServiceFactory>;
+
+  PrefetchServiceFactory();
+  ~PrefetchServiceFactory() override {}
+
+  KeyedService* BuildServiceInstanceFor(
+      content::BrowserContext* context) const override;
+
+  DISALLOW_COPY_AND_ASSIGN(PrefetchServiceFactory);
+};
+
+}  // namespace offline_pages
+
+#endif  // CHROME_BROWSER_ANDROID_OFFLINE_PAGES_PREFETCH_SERVICE_FACTORY_H_