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

sync/api/fake_syncable_service.cc

Index: sync/api/fake_syncable_service.cc
diff --git a/sync/api/fake_syncable_service.cc b/sync/api/fake_syncable_service.cc
index ec7b90cf53be24bd54de84b09e4988d835d2f66b..5969d34174b637aa2fb65f2fa0c02bac2512d3b1 100644
--- a/sync/api/fake_syncable_service.cc
+++ b/sync/api/fake_syncable_service.cc
@@ -26,8 +26,8 @@ void FakeSyncableService::set_process_sync_changes_error(
 }
 
 void FakeSyncableService::set_attachment_store(
-    const scoped_refptr<AttachmentStore>& attachment_store) {
-  attachment_store_ = attachment_store;
+    scoped_ptr<AttachmentStore> attachment_store) {
+  attachment_store_ = attachment_store.Pass();
 }
 
 const AttachmentService* FakeSyncableService::attachment_service() const {
@@ -70,8 +70,8 @@ SyncError FakeSyncableService::ProcessSyncChanges(
   return process_sync_changes_error_;
 }
 
-scoped_refptr<AttachmentStore> FakeSyncableService::GetAttachmentStore() {
-  return attachment_store_;
+AttachmentStore* FakeSyncableService::GetAttachmentStore() {
+  return attachment_store_.get();
 }
 
 void FakeSyncableService::SetAttachmentService(