Cleanup: Remove unneeded forward declarations in chrome/browser/sync.

chrome/browser/sync/glue/history_model_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_GLUE_HISTORY_MODEL_WORKER_H_
 #define CHROME_BROWSER_SYNC_GLUE_HISTORY_MODEL_WORKER_H_
 #pragma once
 
 #include "chrome/browser/sync/engine/model_safe_worker.h"
 #include "chrome/browser/sync/util/unrecoverable_error_info.h"
 
 #include "base/basictypes.h"
 #include "base/callback.h"
 #include "base/memory/ref_counted.h"
 #include "content/browser/cancelable_request.h"
 
 class HistoryService;
 
-namespace base {
-class WaitableEvent;
-}
-
 namespace browser_sync {
 
 // A ModelSafeWorker for history models that accepts requests
 // from the syncapi that need to be fulfilled on the history thread.
 class HistoryModelWorker : public browser_sync::ModelSafeWorker {
  public:
   explicit HistoryModelWorker(HistoryService* history_service);
   virtual ~HistoryModelWorker();
 
   // ModelSafeWorker implementation. Called on syncapi SyncerThread.
   virtual UnrecoverableErrorInfo DoWorkAndWaitUntilDone(
       const WorkCallback& work) OVERRIDE;
   virtual ModelSafeGroup GetModelSafeGroup() OVERRIDE;
 
  private:
   scoped_refptr<HistoryService> history_service_;
   // Helper object to make sure we don't leave tasks running on the history
   // thread.
   CancelableRequestConsumerT<int, 0> cancelable_consumer_;
   DISALLOW_COPY_AND_ASSIGN(HistoryModelWorker);
 };
 
 }  // namespace browser_sync
 
 #endif  // CHROME_BROWSER_SYNC_GLUE_HISTORY_MODEL_WORKER_H_