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

sync/internal_api/public/attachments/attachment_store_handle.h

-// Copyright 2014 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.
-
-#ifndef SYNC_INTERNAL_API_PUBLIC_ATTACHMENTS_ATTACHMENT_STORE_HANDLE_H_
-#define SYNC_INTERNAL_API_PUBLIC_ATTACHMENTS_ATTACHMENT_STORE_HANDLE_H_
-
-#include <map>
-
-#include "base/memory/ref_counted.h"
-#include "base/memory/scoped_ptr.h"
-#include "base/stl_util.h"
-#include "base/threading/non_thread_safe.h"
-#include "sync/api/attachments/attachment_store.h"
-#include "sync/base/sync_export.h"
-
-namespace base {
-class SequencedTaskRunner;
-}  // namespace base
-
-namespace sync_pb {
-class AttachmentId;
-}  // namespace sync_pb
-
-namespace syncer {
-
-class Attachment;
-
-// AttachmentStoreHandle is helper to post AttachmentStore calls to backend on
-// different thread. Backend is expected to know on which thread to post
-// callbacks with results.
-// AttachmentStoreHandle takes ownership of backend. Backend is deleted on
-// backend thread.
-// AttachmentStoreHandle is not thread safe, it should only be accessed from
-// model thread.
-class SYNC_EXPORT AttachmentStoreHandle : public AttachmentStore,
-                                          public base::NonThreadSafe {
- public:
-  AttachmentStoreHandle(
-      scoped_ptr<AttachmentStoreBackend> backend,
-      const scoped_refptr<base::SequencedTaskRunner>& backend_task_runner);
-
-  // AttachmentStore implementation.
-  void Init(const InitCallback& callback) override;
-  void Read(const AttachmentIdList& id, const ReadCallback& callback) override;
-  void Write(const AttachmentList& attachments,
-             const WriteCallback& callback) override;
-  void Drop(const AttachmentIdList& id, const DropCallback& callback) override;
-  void ReadMetadata(const AttachmentIdList& ids,
-                    const ReadMetadataCallback& callback) override;
-  void ReadAllMetadata(const ReadMetadataCallback& callback) override;
-
- private:
-  ~AttachmentStoreHandle() override;
-
-  // AttachmentStoreHandle controls backend's lifetime. It is safe for
-  // AttachmentStoreHandle to bind backend through base::Unretained for posts.
-  // Backend is deleted on backend_task_runner, after that backend_ pointer is
-  // invalid.
-  scoped_ptr<AttachmentStoreBackend> backend_;
-  scoped_refptr<base::SequencedTaskRunner> backend_task_runner_;
-
-  DISALLOW_COPY_AND_ASSIGN(AttachmentStoreHandle);
-};
-
-}  // namespace syncer
-
-#endif  // SYNC_INTERNAL_API_PUBLIC_ATTACHMENTS_ATTACHMENT_STORE_HANDLE_H_