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

sync/api/fake_syncable_service.h

 // 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.
 
 #ifndef SYNC_API_FAKE_SYNCABLE_SERVICE_H_
 #define SYNC_API_FAKE_SYNCABLE_SERVICE_H_
 
 #include "sync/api/syncable_service.h"
 
 namespace syncer {
 
 class SyncErrorFactory;
 
 // A fake SyncableService that can return arbitrary values and maintains the
 // syncing status.
 class FakeSyncableService : public SyncableService {
  public:
   FakeSyncableService();
   ~FakeSyncableService() override;
 
   // Setters for SyncableService implementation results.
   void set_merge_data_and_start_syncing_error(const SyncError& error);
   void set_process_sync_changes_error(const SyncError& error);
 
   // Setter for AttachmentStore.
-  void set_attachment_store(
-      const scoped_refptr<AttachmentStore>& attachment_store);
+  void set_attachment_store(scoped_ptr<AttachmentStore> attachment_store);
 
   // AttachmentService should be set when this syncable service is connected,
   // just before MergeDataAndStartSyncing. NULL is returned by default.
   const AttachmentService* attachment_service() const;
 
   // Whether we're syncing or not. Set on a successful MergeDataAndStartSyncing,
   // unset on StopSyncing. False by default.
   bool syncing() const;
 
   // SyncableService implementation.
   SyncMergeResult MergeDataAndStartSyncing(
       ModelType type,
       const SyncDataList& initial_sync_data,
       scoped_ptr<SyncChangeProcessor> sync_processor,
       scoped_ptr<SyncErrorFactory> sync_error_factory) override;
   void StopSyncing(ModelType type) override;
   SyncDataList GetAllSyncData(ModelType type) const override;
   SyncError ProcessSyncChanges(const tracked_objects::Location& from_here,
                                const SyncChangeList& change_list) override;
-  scoped_refptr<AttachmentStore> GetAttachmentStore() override;
+  scoped_ptr<AttachmentStore> GetAttachmentStoreForSync() override;
   void SetAttachmentService(
       scoped_ptr<AttachmentService> attachment_service) override;
 
  private:
   scoped_ptr<SyncChangeProcessor> sync_processor_;
   SyncError merge_data_and_start_syncing_error_;
   SyncError process_sync_changes_error_;
   bool syncing_;
   ModelType type_;
-  scoped_refptr<AttachmentStore> attachment_store_;
+  scoped_ptr<AttachmentStore> attachment_store_;
   scoped_ptr<AttachmentService> attachment_service_;
 };
 
 }  // namespace syncer
 
 #endif  // SYNC_API_FAKE_SYNCABLE_SERVICE_H_