Add AttachmentDownloader interface, change signature of AttachmentStore::Read

sync/api/attachments/attachment_service_impl.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_API_ATTACHMENTS_ATTACHMENT_SERVICE_IMPL_H_
 #define SYNC_API_ATTACHMENTS_ATTACHMENT_SERVICE_IMPL_H_
 
 #include "base/memory/weak_ptr.h"
 #include "base/threading/non_thread_safe.h"
 #include "sync/api/attachments/attachment_service.h"
@@ skipping 27 matching lines @@
                                const DropCallback& callback) OVERRIDE;
   virtual void StoreAttachments(const AttachmentList& attachments,
                                 const StoreCallback& callback) OVERRIDE;
   virtual void OnSyncDataDelete(const SyncData& sync_data) OVERRIDE;
   virtual void OnSyncDataUpdate(const AttachmentIdList& old_attachment_ids,
                                 const SyncData& updated_sync_data) OVERRIDE;
 
  private:
   void ReadDone(const GetOrDownloadCallback& callback,
                 const AttachmentStore::Result& result,
-                scoped_ptr<AttachmentMap> attachments);
+                scoped_ptr<AttachmentMap> attachments,
+                scoped_ptr<AttachmentIdList> unavailable_attachment_ids);
   void DropDone(const DropCallback& callback,
                 const AttachmentStore::Result& result);
   void WriteDone(const StoreCallback& callback,
                  const AttachmentStore::Result& result);
   void UploadDone(const AttachmentUploader::UploadResult& result,
                   const AttachmentId& attachment_id);
 
   const scoped_ptr<AttachmentStore> attachment_store_;
   const scoped_ptr<AttachmentUploader> attachment_uploader_;
   Delegate* delegate_;
   // Must be last data member.
   base::WeakPtrFactory<AttachmentServiceImpl> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(AttachmentServiceImpl);
 };
 
 }  // namespace syncer
 
 #endif  // SYNC_API_ATTACHMENTS_ATTACHMENT_SERVICE_IMPL_H_