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

components/sync/user_events/no_op_user_event_service.h

Index: components/sync/user_events/no_op_user_event_service.h
diff --git a/components/sync/user_events/no_op_user_event_service.h b/components/sync/user_events/no_op_user_event_service.h
new file mode 100644
index 0000000000000000000000000000000000000000..2d69045377908e2e1988a9e49060b004ffcea292
--- /dev/null
+++ b/components/sync/user_events/no_op_user_event_service.h
@@ -0,0 +1,40 @@
+// 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_NO_OP_USER_EVENT_SERVICE_H_
+#define COMPONENTS_SYNC_USER_EVENTS_NO_OP_USER_EVENT_SERVICE_H_
+
+#include <memory>
+#include <string>
+
+#include "base/macros.h"
+#include "components/sync/user_events/user_event_service.h"
+
+namespace syncer {
+
+class ModelTypeSyncBridge;
+
+// This implementation is used when we know event should never be recorded,
+// such as in incognito mode.
+class NoOpUserEventService : public UserEventService {
+ public:
+  NoOpUserEventService();
+  ~NoOpUserEventService() override;
+
+  // UserEventService implementation.
+  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;
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(NoOpUserEventService);
+};
+
+}  // namespace syncer
+
+#endif  // COMPONENTS_SYNC_USER_EVENTS_NO_OP_USER_EVENT_SERVICE_H_