Move the message definitions to their own files.

xfa/fwl/core/cfwl_message.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_message.h"
+
+CFWL_Message::CFWL_Message()
+    : m_pSrcTarget(nullptr), m_pDstTarget(nullptr), m_dwExtend(0) {}
+
+CFWL_Message::~CFWL_Message() {}
+
+std::unique_ptr<CFWL_Message> CFWL_Message::Clone() {
+  return nullptr;
+}
+
+CFWL_MessageType CFWL_Message::GetClassID() const {
+  return CFWL_MessageType::None;
+}