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

sync/internal_api/public/attachments/attachment_store_frontend.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_INTERNAL_API_PUBLIC_ATTACHMENTS_ATTACHMENT_STORE_FRONTEND_H_
-#define SYNC_INTERNAL_API_PUBLIC_ATTACHMENTS_ATTACHMENT_STORE_FRONTEND_H_
-
-#include "base/memory/ref_counted.h"
-#include "base/memory/scoped_ptr.h"
-#include "base/threading/non_thread_safe.h"
-#include "sync/api/attachments/attachment_store.h"
-
-namespace base {
-class SequencedTaskRunner;
-}  // namespace base
-
-namespace syncer {
-
-class AttachmentStoreBackend;
-
-// AttachmentStoreFrontend is helper to post AttachmentStore calls to backend on
-// different thread. Backend is expected to know on which thread to post
-// callbacks with results.
-// AttachmentStoreFrontend takes ownership of backend. Backend is deleted on
-// backend thread.
-// AttachmentStoreFrontend is not thread safe, it should only be accessed from
-// model thread.
-// Method signatures of AttachmentStoreFrontend match exactly methods of
-// AttachmentStoreBackend.
-class SYNC_EXPORT AttachmentStoreFrontend
-    : public base::RefCounted<AttachmentStoreFrontend>,
-      public base::NonThreadSafe {
- public:
-  AttachmentStoreFrontend(
-      scoped_ptr<AttachmentStoreBackend> backend,
-      const scoped_refptr<base::SequencedTaskRunner>& backend_task_runner);
-
-  void Init(const AttachmentStore::InitCallback& callback);
-
-  void Read(const AttachmentIdList& ids,
-            const AttachmentStore::ReadCallback& callback);
-
-  void Write(AttachmentStore::AttachmentReferrer referrer,
-             const AttachmentList& attachments,
-             const AttachmentStore::WriteCallback& callback);
-
-  void Drop(AttachmentStore::AttachmentReferrer referrer,
-            const AttachmentIdList& ids,
-            const AttachmentStore::DropCallback& callback);
-
-  void ReadMetadata(const AttachmentIdList& ids,
-                    const AttachmentStore::ReadMetadataCallback& callback);
-
-  void ReadAllMetadata(AttachmentStore::AttachmentReferrer referrer,
-                       const AttachmentStore::ReadMetadataCallback& callback);
-
- private:
-  friend class base::RefCounted<AttachmentStoreFrontend>;
-  virtual ~AttachmentStoreFrontend();
-
-  scoped_ptr<AttachmentStoreBackend> backend_;
-  scoped_refptr<base::SequencedTaskRunner> backend_task_runner_;
-
-  DISALLOW_COPY_AND_ASSIGN(AttachmentStoreFrontend);
-};
-
-}  // namespace syncer
-
-#endif  // SYNC_INTERNAL_API_PUBLIC_ATTACHMENTS_ATTACHMENT_STORE_FRONTEND_H_