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

sync/internal_api/attachments/attachment_store_test_template.h

Index: sync/internal_api/attachments/attachment_store_test_template.h
diff --git a/sync/internal_api/attachments/attachment_store_test_template.h b/sync/internal_api/attachments/attachment_store_test_template.h
index 290ef220573747d59d76dcc7827e1090dbe8d25d..0d14ca542155fecead3bb02f71dbe531c313771b 100644
--- a/sync/internal_api/attachments/attachment_store_test_template.h
+++ b/sync/internal_api/attachments/attachment_store_test_template.h
@@ -47,7 +47,7 @@ class AttachmentStoreTest : public testing::Test {
  protected:
   AttachmentStoreFactory attachment_store_factory;
   base::MessageLoop message_loop;
-  scoped_refptr<AttachmentStore> store;
+  scoped_ptr<AttachmentStore> store;
   AttachmentStore::Result result;
   scoped_ptr<AttachmentMap> attachments;
   scoped_ptr<AttachmentIdList> failed_attachment_ids;