[Sync] Refactor AttachmentStore classes. Introduce concept of referrer.

sync/api/syncable_service.cc

 // Copyright (c) 2012 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 "sync/api/syncable_service.h"
 
 namespace syncer {
 
 SyncableService::~SyncableService() {}
 
-scoped_refptr<AttachmentStore> SyncableService::GetAttachmentStore() {
-  return scoped_refptr<AttachmentStore>();
+AttachmentStore* SyncableService::GetAttachmentStore() {
+  return nullptr;
 }
 
 void SyncableService::SetAttachmentService(
     scoped_ptr<AttachmentService> attachment_service) {
 }
 
 }  // namespace syncer