[sync] Componentize sync: Part 6: Add more SYNC_EXPORTs to files in src/sync/

sync/notifier/state_writer.h

-// Copyright (c) 2012 The Chromium Authors. All rights reserved.
+// 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.
 //
 // Simple interface for something that persists state.
 
 #ifndef SYNC_NOTIFIER_STATE_WRITER_H_
 #define SYNC_NOTIFIER_STATE_WRITER_H_
 
 #include <string>
 
+#include "sync/base/sync_export.h"
+
 namespace syncer {
 
-class StateWriter {
+class SYNC_EXPORT_PRIVATE StateWriter {
  public:
   virtual ~StateWriter() {}
 
   virtual void WriteState(const std::string& state) = 0;
 };
 
 }  // namespace syncer
 
 #endif  // SYNC_NOTIFIER_STATE_WRITER_H_