[Sync] Create iOS factory for UserEventService.

ios/chrome/browser/sync/ios_user_event_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 IOS_CHROME_BROWSER_SYNC_IOS_USER_EVENT_SERVICE_FACTORY_H_
+#define IOS_CHROME_BROWSER_SYNC_IOS_USER_EVENT_SERVICE_FACTORY_H_
+
+#include <memory>
+
+#include "base/macros.h"
+#include "components/keyed_service/ios/browser_state_keyed_service_factory.h"
+
+namespace base {
+template <typename T>
+struct DefaultSingletonTraits;
+}  // namespace base
+
+namespace ios {
+class ChromeBrowserState;
+}  // namespace ios
+
+namespace syncer {
+class UserEventService;
+}  // namespace syncer
+
+// Singleton that associates UserEventServices to ChromeBrowserStates.
+class IOSUserEventServiceFactory : public BrowserStateKeyedServiceFactory {
+ public:
+  static syncer::UserEventService* GetForBrowserState(
+      ios::ChromeBrowserState* browser_state);
+
+  static IOSUserEventServiceFactory* GetInstance();
+
+ private:
+  friend struct base::DefaultSingletonTraits<IOSUserEventServiceFactory>;
+
+  IOSUserEventServiceFactory();
+  ~IOSUserEventServiceFactory() override;
+
+  // BrowserStateKeyedServiceFactory implementation.
+  std::unique_ptr<KeyedService> BuildServiceInstanceFor(
+      web::BrowserState* context) const override;
+  web::BrowserState* GetBrowserStateToUse(
+      web::BrowserState* context) const override;
+};
+
+#endif  // IOS_CHROME_BROWSER_SYNC_IOS_USER_EVENT_SERVICE_FACTORY_H_