iwyu: Use callback_old.h where appropriate, part 2.

chrome/browser/sync/engine/model_safe_worker.h

 // Copyright (c) 2011 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 CHROME_BROWSER_SYNC_ENGINE_MODEL_SAFE_WORKER_H_
 #define CHROME_BROWSER_SYNC_ENGINE_MODEL_SAFE_WORKER_H_
 #pragma once
 
 #include <map>
 #include <string>
 #include <vector>
 
-#include "base/callback.h"
+#include "base/callback_old.h"
 #include "base/memory/ref_counted.h"
 #include "chrome/browser/sync/syncable/model_type.h"
 
 namespace browser_sync {
 
 enum ModelSafeGroup {
   GROUP_PASSIVE = 0,   // Models that are just "passively" being synced; e.g.
                        // changes to these models don't need to be pushed to a
                        // native model.
   GROUP_UI,            // Models that live on UI thread and are being synced.
@@ skipping 69 matching lines @@
   virtual void GetModelSafeRoutingInfo(ModelSafeRoutingInfo* out) = 0;
  protected:
   virtual ~ModelSafeWorkerRegistrar() {}
  private:
   DISALLOW_COPY_AND_ASSIGN(ModelSafeWorkerRegistrar);
 };
 
 }  // namespace browser_sync
 
 #endif  // CHROME_BROWSER_SYNC_ENGINE_MODEL_SAFE_WORKER_H_