Remove #pragma once from sync

sync/syncable/transaction_observer.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_SYNCABLE_TRANSACTION_OBSERVER_H_
 #define SYNC_SYNCABLE_TRANSACTION_OBSERVER_H_
-#pragma once
 
 #include "sync/internal_api/public/base/model_type.h"
 #include "sync/syncable/write_transaction_info.h"
 
 namespace syncer {
 namespace syncable {
 
 class TransactionObserver {
  public:
   virtual void OnTransactionWrite(
       const ImmutableWriteTransactionInfo& write_transaction_info,
       ModelTypeSet models_with_changes) = 0;
  protected:
   virtual ~TransactionObserver() {}
 };
 
 }  // namespace syncable
 }  // namespace syncer
 
 #endif  // SYNC_SYNCABLE_TRANSACTION_OBSERVER_H_