sync: Process 'control' data separately

sync/engine/apply_metadata_updates.h

+// Copyright (c) 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_ENGINE_APPLY_METADATA_UPDATES_H_
+#define SYNC_ENGINE_APPLY_METADATA_UPDATES_H_
+#pragma once
+
+namespace syncable {
+class Directory;
+class WriteTransaction;
+}
+
+namespace browser_sync {
+
+class Cryptographer;
+
+void ApplyMetadataUpdates(syncable::Directory* dir);
+void ApplyNigoriUpdates(syncable::WriteTransaction* trans,
+                        Cryptographer* cryptographer);
+
+}
+
+#endif  // SYNC_ENGINE_APPLY_METADATA_UPDATES_H_