Cleanup FWL Event and Message code.

xfa/fwl/core/cfwl_msgmouse.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_MSGMOUSE_H_
 #define XFA_FWL_CORE_CFWL_MSGMOUSE_H_
 
 #include <memory>
 
 #include "xfa/fwl/core/cfwl_message.h"
 
 enum class FWL_MouseCommand {
   LeftButtonDown,
   LeftButtonUp,
   LeftButtonDblClk,
   RightButtonDown,
   RightButtonUp,
   RightButtonDblClk,
   Move,
   Enter,
   Leave,
   Hover
 };
 
 class CFWL_MsgMouse : public CFWL_Message {
  public:
-  CFWL_MsgMouse();
+  CFWL_MsgMouse(CFWL_Widget* pSrcTarget, CFWL_Widget* pDstTarget);
   ~CFWL_MsgMouse() override;
 
   // CFWL_Message
   std::unique_ptr<CFWL_Message> Clone() override;
-  CFWL_MessageType GetClassID() const override;
 
   FX_FLOAT m_fx;
   FX_FLOAT m_fy;
   uint32_t m_dwFlags;
   FWL_MouseCommand m_dwCmd;
 };
 
 #endif  // XFA_FWL_CORE_CFWL_MSGMOUSE_H_