Cleanup FWL Event and Message code.

xfa/fwl/core/cfwl_msgmousewheel.cpp

 // 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
 
 #include "xfa/fwl/core/cfwl_msgmousewheel.h"
 
 #include <memory>
 
 #include "third_party/base/ptr_util.h"
 
-CFWL_MsgMouseWheel::CFWL_MsgMouseWheel() {}
+CFWL_MsgMouseWheel::CFWL_MsgMouseWheel()
+    : CFWL_Message(CFWL_MessageType::MouseWheel) {}
 
 CFWL_MsgMouseWheel::~CFWL_MsgMouseWheel() {}
 
 std::unique_ptr<CFWL_Message> CFWL_MsgMouseWheel::Clone() {
   return pdfium::MakeUnique<CFWL_MsgMouseWheel>(*this);
 }
-
-CFWL_MessageType CFWL_MsgMouseWheel::GetClassID() const {
-  return CFWL_MessageType::MouseWheel;
-}