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

sync/api/attachments/attachment_store_backend.cc

Index: sync/api/attachments/attachment_store_backend.cc
diff --git a/sync/api/attachments/attachment_store_backend.cc b/sync/api/attachments/attachment_store_backend.cc
new file mode 100644
index 0000000000000000000000000000000000000000..44363cc1a01a6087409f47e6f81aa48b680e4edd
--- /dev/null
+++ b/sync/api/attachments/attachment_store_backend.cc
@@ -0,0 +1,24 @@
+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "sync/api/attachments/attachment_store_backend.h"
+
+#include "base/location.h"
+#include "base/sequenced_task_runner.h"
+
+namespace syncer {
+
+AttachmentStoreBackend::AttachmentStoreBackend(
+    const scoped_refptr<base::SequencedTaskRunner>& callback_task_runner)
+    : callback_task_runner_(callback_task_runner) {
+}
+
+AttachmentStoreBackend::~AttachmentStoreBackend() {
+}
+
+void AttachmentStoreBackend::PostCallback(const base::Closure& callback) {
+  callback_task_runner_->PostTask(FROM_HERE, callback);
+}
+
+}  // namespace syncer