[Sync] Introduce AttachmentStoreForSync class

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 d2177e9a0c1f0668e5160ab86e6dbae33acc0d99..342dd2db008d2ecafa6c03fffa6a0596567dcdf5 100644
--- a/sync/api/fake_syncable_service.cc
+++ b/sync/api/fake_syncable_service.cc
@@ -70,9 +70,10 @@ SyncError FakeSyncableService::ProcessSyncChanges(
   return process_sync_changes_error_;
 }
 
-scoped_ptr<AttachmentStore> FakeSyncableService::GetAttachmentStoreForSync() {
+scoped_ptr<AttachmentStoreForSync>
+FakeSyncableService::GetAttachmentStoreForSync() {
   return attachment_store_ ? attachment_store_->CreateAttachmentStoreForSync()
-                           : scoped_ptr<AttachmentStore>();
+                           : scoped_ptr<AttachmentStoreForSync>();
 }
 
 void FakeSyncableService::SetAttachmentService(