Get rid of NULLs in xfa/

xfa/fxfa/include/xfa_rendercontext.h

 // Copyright 2014 PDFium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Original code copyright 2014 Foxit Software Inc. http://www.foxitsoftware.com
 
 #ifndef XFA_FXFA_INCLUDE_XFA_RENDERCONTEXT_H_
 #define XFA_FXFA_INCLUDE_XFA_RENDERCONTEXT_H_
 
 #include <memory>
 
 #include "xfa/fxfa/include/fxfa.h"
 
 class CXFA_RenderContext {
  public:
   CXFA_RenderContext();
   ~CXFA_RenderContext();
 
   int32_t StartRender(CXFA_FFPageView* pPageView,
                       CFX_Graphics* pGS,
                       const CFX_Matrix& matrix,
                       const CXFA_RenderOptions& options);
-  int32_t DoRender(IFX_Pause* pPause = NULL);
+  int32_t DoRender(IFX_Pause* pPause = nullptr);
   void StopRender();
 
  protected:
   std::unique_ptr<IXFA_WidgetIterator> m_pWidgetIterator;
   CXFA_FFWidget* m_pWidget;
   CXFA_FFPageView* m_pPageView;
   CFX_Graphics* m_pGS;
   CFX_Matrix m_matrix;
   CXFA_RenderOptions m_options;
   uint32_t m_dwStatus;
   CFX_RectF m_rtClipRect;
 };
 
 #endif  // XFA_FXFA_INCLUDE_XFA_RENDERCONTEXT_H_