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

sync/internal_api/attachments/in_memory_attachment_store_unittest.cc

 // Copyright 2014 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/attachments/attachment_store.h"
 
 #include "sync/internal_api/attachments/attachment_store_test_template.h"
 
 namespace syncer {
 
 class InMemoryAttachmentStoreFactory {
  public:
   InMemoryAttachmentStoreFactory() {}
   ~InMemoryAttachmentStoreFactory() {}
 
-  scoped_refptr<AttachmentStore> CreateAttachmentStore() {
+  scoped_ptr<AttachmentStore> CreateAttachmentStore() {
     return AttachmentStore::CreateInMemoryStore();
   }
 };
 
 INSTANTIATE_TYPED_TEST_CASE_P(InMemory,
                               AttachmentStoreTest,
                               InMemoryAttachmentStoreFactory);
 
 }  // namespace syncer