Cleanup FWL default values part II.

xfa/fwl/core/ifwl_widgetdelegate.h

 // Copyright 2016 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_FWL_CORE_IFWL_WIDGETDELEGATE_H_
 #define XFA_FWL_CORE_IFWL_WIDGETDELEGATE_H_
 
 #include <stdint.h>
 
 class CFWL_Event;
 class CFWL_Message;
 class CFX_Graphics;
 class CFX_Matrix;
 
 class IFWL_WidgetDelegate {
  public:
   virtual ~IFWL_WidgetDelegate() {}
 
   virtual void OnProcessMessage(CFWL_Message* pMessage) = 0;
   virtual void OnProcessEvent(CFWL_Event* pEvent) = 0;
   virtual void OnDrawWidget(CFX_Graphics* pGraphics,
-                            const CFX_Matrix* pMatrix = nullptr) = 0;
+                            const CFX_Matrix* pMatrix) = 0;
 };
 
 #endif  // XFA_FWL_CORE_IFWL_WIDGETDELEGATE_H_