Cleanup FWL Event and Message code.

xfa/fwl/core/cfwl_msgmousewheel.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_CFWL_MSGMOUSEWHEEL_H_
 #define XFA_FWL_CORE_CFWL_MSGMOUSEWHEEL_H_
 
 #include <memory>
 
 #include "xfa/fwl/core/cfwl_message.h"
 
 class CFWL_MsgMouseWheel : public CFWL_Message {
  public:
-  CFWL_MsgMouseWheel();
+  CFWL_MsgMouseWheel(CFWL_Widget* pSrcTarget, CFWL_Widget* pDstTarget);
   ~CFWL_MsgMouseWheel() override;
 
   // CFWL_Message
   std::unique_ptr<CFWL_Message> Clone() override;
-  CFWL_MessageType GetClassID() const override;
 
   FX_FLOAT m_fx;
   FX_FLOAT m_fy;
   FX_FLOAT m_fDeltaX;
   FX_FLOAT m_fDeltaY;
   uint32_t m_dwFlags;
 };
 
 #endif  // XFA_FWL_CORE_CFWL_MSGMOUSEWHEEL_H_