FBTF: Remove unneeded headers from base/ (part 2)

chrome/browser/sync/glue/database_model_worker.h

 // Copyright (c) 2010 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_GLUE_DATABASE_MODEL_WORKER_H_
 #define CHROME_BROWSER_SYNC_GLUE_DATABASE_MODEL_WORKER_H_
 #pragma once
 
 #include "base/callback.h"
 #include "base/ref_counted.h"
-#include "base/waitable_event.h"
 #include "chrome/browser/sync/engine/model_safe_worker.h"
 
+namespace base {
+class WaitableEvent;
+}
+
 namespace browser_sync {
 
 // A ModelSafeWorker for database models (eg. autofill) that accepts requests
 // from the syncapi that need to be fulfilled on the database thread.
 class DatabaseModelWorker : public browser_sync::ModelSafeWorker {
  public:
   explicit DatabaseModelWorker() {}
 
   // ModelSafeWorker implementation. Called on syncapi SyncerThread.
   void DoWorkAndWaitUntilDone(Callback0::Type* work);
   virtual ModelSafeGroup GetModelSafeGroup() { return GROUP_DB; }
   virtual bool CurrentThreadIsWorkThread();
 
  private:
   void CallDoWorkAndSignalTask(Callback0::Type* work,
                                base::WaitableEvent* done);
 
   DISALLOW_COPY_AND_ASSIGN(DatabaseModelWorker);
 };
 
 }  // namespace browser_sync
 
 #endif  // CHROME_BROWSER_SYNC_GLUE_DATABASE_MODEL_WORKER_H_