[Sync] Introduce AttachmentStoreForSync class

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"
@@ skipping 22 matching lines @@
  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,
+  void Write(AttachmentStore::Component component,
              const AttachmentList& attachments,
              const AttachmentStore::WriteCallback& callback);
-
-  void Drop(AttachmentStore::AttachmentReferrer referrer,
-            const AttachmentIdList& ids,
-            const AttachmentStore::DropCallback& callback);
+  void SetReference(AttachmentStore::Component component,
+                    const AttachmentIdList& ids);
+  void DropReference(AttachmentStore::Component component,
+                     const AttachmentIdList& ids,
+                     const AttachmentStore::DropCallback& callback);
 
   void ReadMetadata(const AttachmentIdList& ids,
                     const AttachmentStore::ReadMetadataCallback& callback);
 
-  void ReadAllMetadata(AttachmentStore::AttachmentReferrer referrer,
+  void ReadAllMetadata(AttachmentStore::Component component,
                        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_