Move the message definitions to their own files.

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 "third_party/base/ptr_util.h"
+
+CFWL_MsgMouseWheel::CFWL_MsgMouseWheel() {}
+
+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;
+}