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

components/sync/user_events/no_op_user_event_service.cc

+// 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.
+
+#include "components/sync/user_events/no_op_user_event_service.h"
+
+#include "base/memory/weak_ptr.h"
+
+using sync_pb::UserEventSpecifics;
+
+namespace syncer {
+
+NoOpUserEventService::NoOpUserEventService() {}
+
+NoOpUserEventService::~NoOpUserEventService() {}
+
+void NoOpUserEventService::RecordUserEvent(
+    std::unique_ptr<UserEventSpecifics> specifics) {}
+
+void NoOpUserEventService::RecordUserEvent(
+    const UserEventSpecifics& specifics) {}
+
+base::WeakPtr<ModelTypeSyncBridge> NoOpUserEventService::GetSyncBridge() {
+  return base::WeakPtr<ModelTypeSyncBridge>();
+}
+
+void NoOpUserEventService::RegisterDependentFieldTrial(
+    const std::string& trial_name,
+    UserEventSpecifics::EventCase event_case) {}
+
+}  // namespace syncer