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

sync/api/syncable_service.cc

Index: sync/api/syncable_service.cc
diff --git a/sync/api/syncable_service.cc b/sync/api/syncable_service.cc
index 7b0a9730a077080a6865a69fd595826257cf8c74..a544367458ff253adf6cfb58a9595a293efb1e59 100644
--- a/sync/api/syncable_service.cc
+++ b/sync/api/syncable_service.cc
@@ -8,8 +8,8 @@ namespace syncer {
 
 SyncableService::~SyncableService() {}
 
-scoped_refptr<AttachmentStore> SyncableService::GetAttachmentStore() {
-  return scoped_refptr<AttachmentStore>();
+AttachmentStore* SyncableService::GetAttachmentStore() {
+  return nullptr;
 }
 
 void SyncableService::SetAttachmentService(