[Sync] Split UserEventService into interface and impl, add fake impl, add unit tests.

components/sync/user_events/fake_user_event_service.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 COMPONENTS_SYNC_USER_EVENTS_FAKE_USER_EVENT_SERVICE_H_
+#define COMPONENTS_SYNC_USER_EVENTS_FAKE_USER_EVENT_SERVICE_H_
+
+#include <map>
+#include <memory>
+#include <set>
+#include <string>
+#include <vector>
+
+#include "base/macros.h"
+#include "base/memory/weak_ptr.h"
+#include "components/sync/protocol/user_event_specifics.pb.h"
+#include "components/sync/user_events/user_event_service.h"
+
+namespace syncer {
+
+class ModelTypeSyncBridge;
+
+class FakeUserEventService : public UserEventService {

[Patrick Noland, 2017/05/30 23:06:41]

Can you add a short comment explaining why/how one would use the
FakeUserEventService? 

[skym, 2017/05/30 23:21:39]

Done.

+ public:
+  FakeUserEventService();
+  ~FakeUserEventService() override;
+
+  // FakeUserEventService implementation.

[Patrick Noland, 2017/05/30 23:06:41]

I think you mean UserEventService implementation?

[skym, 2017/05/30 23:21:39]

Yeah, search/replace fail, done.

+  void RecordUserEvent(
+      std::unique_ptr<sync_pb::UserEventSpecifics> specifics) override;
+  void RecordUserEvent(const sync_pb::UserEventSpecifics& specifics) override;
+  void RegisterDependentFieldTrial(
+      const std::string& trial_name,
+      sync_pb::UserEventSpecifics::EventCase event_case) override;
+  base::WeakPtr<ModelTypeSyncBridge> GetSyncBridge() override;
+
+  const std::vector<sync_pb::UserEventSpecifics>& GetRecordedUserEvents() const;
+  const std::map<std::string, std::set<sync_pb::UserEventSpecifics::EventCase>>&
+  GetRegisteredDependentFieldTrials() const;
+
+ private:
+  std::vector<sync_pb::UserEventSpecifics> recorded_user_events_;
+  std::map<std::string, std::set<sync_pb::UserEventSpecifics::EventCase>>
+      registered_dependent_field_trials_;
+
+  DISALLOW_COPY_AND_ASSIGN(FakeUserEventService);
+};
+
+}  // namespace syncer
+
+#endif  // COMPONENTS_SYNC_USER_EVENTS_FAKE_USER_EVENT_SERVICE_H_