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

sync/api/attachments/attachment_store_backend.h

-// 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.
-
-#ifndef SYNC_API_ATTACHMENTS_ATTACHMENT_STORE_BACKEND_H_
-#define SYNC_API_ATTACHMENTS_ATTACHMENT_STORE_BACKEND_H_
-
-#include "base/callback.h"
-#include "base/memory/ref_counted.h"
-#include "sync/api/attachments/attachment.h"
-#include "sync/api/attachments/attachment_id.h"
-#include "sync/api/attachments/attachment_store.h"
-#include "sync/base/sync_export.h"
-
-namespace base {
-class SequencedTaskRunner;
-}  // namespace base
-
-namespace syncer {
-
-// Interface for AttachmentStore backends.
-//
-// AttachmentStoreBackend provides interface for different backends (on-disk,
-// in-memory). Factory methods in AttachmentStore create corresponding backend
-// and pass reference to AttachmentStore.
-// All functions in AttachmentStoreBackend mirror corresponding functions in
-// AttachmentStore.
-// All callbacks and result codes are used directly from AttachmentStore.
-// AttachmentStoreFrontend only passes callbacks and results without modifying
-// them, there is no need to declare separate set.
-class SYNC_EXPORT AttachmentStoreBackend {
- public:
-  explicit AttachmentStoreBackend(
-      const scoped_refptr<base::SequencedTaskRunner>& callback_task_runner);
-  virtual ~AttachmentStoreBackend();
-  virtual void Init(const AttachmentStore::InitCallback& callback) = 0;
-  virtual void Read(const AttachmentIdList& ids,
-                    const AttachmentStore::ReadCallback& callback) = 0;
-  virtual void Write(AttachmentStore::AttachmentReferrer referrer,
-                     const AttachmentList& attachments,
-                     const AttachmentStore::WriteCallback& callback) = 0;
-  virtual void Drop(AttachmentStore::AttachmentReferrer referrer,
-                    const AttachmentIdList& ids,
-                    const AttachmentStore::DropCallback& callback) = 0;
-  virtual void ReadMetadata(
-      const AttachmentIdList& ids,
-      const AttachmentStore::ReadMetadataCallback& callback) = 0;
-  virtual void ReadAllMetadata(
-      AttachmentStore::AttachmentReferrer referrer,
-      const AttachmentStore::ReadMetadataCallback& callback) = 0;
-
- protected:
-  // Helper function to post callback on callback_task_runner.
-  void PostCallback(const base::Closure& callback);
-
- private:
-  scoped_refptr<base::SequencedTaskRunner> callback_task_runner_;
-
-  DISALLOW_COPY_AND_ASSIGN(AttachmentStoreBackend);
-};
-
-}  // namespace syncer
-
-#endif  // SYNC_API_ATTACHMENTS_ATTACHMENT_STORE_BACKEND_H_