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

sync/api/fake_syncable_service.h

Index: sync/api/fake_syncable_service.h
diff --git a/sync/api/fake_syncable_service.h b/sync/api/fake_syncable_service.h
index 7403ef3b811864d91ac0c27b04f3f187a03c30da..9b75647abafe443f78a024b271c84cb4d4801da0 100644
--- a/sync/api/fake_syncable_service.h
+++ b/sync/api/fake_syncable_service.h
@@ -23,8 +23,7 @@ class FakeSyncableService : public SyncableService {
   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.
@@ -44,7 +43,7 @@ class FakeSyncableService : public SyncableService {
   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;
 
@@ -54,7 +53,7 @@ class FakeSyncableService : public SyncableService {
   SyncError process_sync_changes_error_;
   bool syncing_;
   ModelType type_;
-  scoped_refptr<AttachmentStore> attachment_store_;
+  scoped_ptr<AttachmentStore> attachment_store_;
   scoped_ptr<AttachmentService> attachment_service_;
 };