Implement a basic Site Engagement Score service.

chrome/browser/engagement/site_engagement_service_factory.h

+// 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.
+
+#ifndef CHROME_BROWSER_ENGAGEMENT_SITE_ENGAGEMENT_SERVICE_FACTORY_H_
+#define CHROME_BROWSER_ENGAGEMENT_SITE_ENGAGEMENT_SERVICE_FACTORY_H_
+
+#include "base/memory/singleton.h"
+#include "components/keyed_service/content/browser_context_keyed_service_factory.h"
+
+class Profile;
+class SiteEngagementService;
+
+// Singleton that owns all SiteEngagementServices and associates them with
+// Profiles. Listens for the Profile's destruction notification and cleans up
+// the associated SiteEngagementService.
+//
+// The default factory behavior is suitable for this factory as:
+// * there should be no site engagement tracking in incognito
+// * the site engagement service should be created lazily
+// * the site engagement service is needed in tests.
+class SiteEngagementServiceFactory : public BrowserContextKeyedServiceFactory {
+ public:
+  static SiteEngagementService* GetForProfile(Profile* profile);
+
+  static SiteEngagementServiceFactory* GetInstance();
+
+ private:
+  friend struct DefaultSingletonTraits<SiteEngagementServiceFactory>;
+
+  SiteEngagementServiceFactory();
+  ~SiteEngagementServiceFactory() override;
+
+  // KeyedServiceBaseFactory:
+  bool ServiceIsNULLWhileTesting() const override;
+
+  // BrowserContextKeyedServiceFactory:
+  KeyedService* BuildServiceInstanceFor(
+      content::BrowserContext* profile) const override;
+};
+
+#endif  // CHROME_BROWSER_ENGAGEMENT_SITE_ENGAGEMENT_SERVICE_FACTORY_H_