[Sync] Introduce AttachmentStoreForSync class

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"
@@ skipping 18 matching lines @@
 // 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,
+  virtual void Write(AttachmentStore::Component component,
                      const AttachmentList& attachments,
                      const AttachmentStore::WriteCallback& callback) = 0;
-  virtual void Drop(AttachmentStore::AttachmentReferrer referrer,
-                    const AttachmentIdList& ids,
-                    const AttachmentStore::DropCallback& callback) = 0;
+  virtual void SetReference(AttachmentStore::Component component,
+                            const AttachmentIdList& ids) = 0;
+  virtual void DropReference(AttachmentStore::Component component,
+                             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,
+      AttachmentStore::Component component,
       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_