[Sync] Introduce AttachmentStoreForSync class

sync/api/syncable_service.h

 // Copyright 2012 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_SYNCABLE_SERVICE_H_
 #define SYNC_API_SYNCABLE_SERVICE_H_
 
 #include <vector>
 
 #include "base/callback.h"
@@ skipping 50 matching lines @@
   // SyncChangeProcessor interface.
   // Process a list of new SyncChanges and update the local data as necessary.
   // Returns: A default SyncError (IsSet() == false) if no errors were
   //          encountered, and a filled SyncError (IsSet() == true)
   //          otherwise.
   SyncError ProcessSyncChanges(const tracked_objects::Location& from_here,
                                const SyncChangeList& change_list) override = 0;
 
   // Returns AttachmentStore for use by sync when uploading or downloading
   // attachments.
-  // GetAttachmentStore is called right before MergeDataAndStartSyncing. If at
-  // that time GetAttachmentStore returns NULL then datatype is considered not
-  // using attachments and all attempts to upload/download attachments will
-  // fail. Default implementation returns NULL. Datatype that uses sync
-  // attachments should create attachment store, implement GetAttachmentStore
-  // to return result of AttachmentStore::CreateAttachmentStoreForSync() from
-  // attachment store object.
-  virtual scoped_ptr<AttachmentStore> GetAttachmentStoreForSync();
+  // GetAttachmentStoreForSync is called right before MergeDataAndStartSyncing.
+  // If at that time GetAttachmentStoreForSync returns NULL then datatype is
+  // considered not using attachments and all attempts to upload/download
+  // attachments will fail. Default implementation returns NULL. Datatype that
+  // uses sync attachments should create attachment store, implement
+  // GetAttachmentStoreForSync to return result of
+  // AttachmentStore::CreateAttachmentStoreForSync() from attachment store
+  // object.
+  virtual scoped_ptr<AttachmentStoreForSync> GetAttachmentStoreForSync();
 
   // Called by sync to provide AttachmentService to be used to download
   // attachments.
   // SetAttachmentService is called after GetAttachmentStore and right before
   // MergeDataAndStartSyncing and only if GetAttachmentStore has returned a
   // non-NULL store instance. Default implementation does nothing.
   // Datatype that uses attachments must take ownerhip of the provided
   // AttachmentService instance.
   virtual void SetAttachmentService(
       scoped_ptr<AttachmentService> attachment_service);
 
  protected:
   ~SyncableService() override;
 };
 
 }  // namespace syncer
 
 #endif  // SYNC_API_SYNCABLE_SERVICE_H_