Convert FWL_ERR into an enum class.

xfa/fwl/lightwidget/cfwl_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_LIGHTWIDGET_CFWL_WIDGETDELEGATE_H_
 #define XFA_FWL_LIGHTWIDGET_CFWL_WIDGETDELEGATE_H_
 
+#include <stdint.h>
+
 #include "xfa/fwl/core/fwl_error.h"
 #include "xfa/fwl/core/ifwl_widgetdelegate.h"
 
 class CFWL_Event;
 class CFWL_Message;
 class CFX_Graphics;
 class CFX_Matrix;
 
 class CFWL_WidgetDelegate : public IFWL_WidgetDelegate {
  public:
   CFWL_WidgetDelegate();
   virtual ~CFWL_WidgetDelegate();
 
-  int32_t OnProcessMessage(CFWL_Message* pMessage) override;
-  FWL_ERR OnProcessEvent(CFWL_Event* pEvent) override;
-  FWL_ERR OnDrawWidget(CFX_Graphics* pGraphics,
-                       const CFX_Matrix* pMatrix = nullptr) override;
+  void OnProcessMessage(CFWL_Message* pMessage) override;
+  void OnProcessEvent(CFWL_Event* pEvent) override;
+  void OnDrawWidget(CFX_Graphics* pGraphics,
+                    const CFX_Matrix* pMatrix = nullptr) override;
 };
 
 #endif  // XFA_FWL_LIGHTWIDGET_CFWL_WIDGETDELEGATE_H_