Move the message definitions to their own files.

xfa/fwl/core/cfwl_msgkey.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 "core/fpdfapi/parser/cpdf_null.h"
+#include "xfa/fwl/core/cfwl_msgkey.h"
+
 #include "third_party/base/ptr_util.h"
 
-CPDF_Null::CPDF_Null() {}
+CFWL_MsgKey::CFWL_MsgKey() {}
 
-CPDF_Object::Type CPDF_Null::GetType() const {
-  return NULLOBJ;
+CFWL_MsgKey::~CFWL_MsgKey() {}
+
+std::unique_ptr<CFWL_Message> CFWL_MsgKey::Clone() {
+  return pdfium::MakeUnique<CFWL_MsgKey>(*this);
 }
 
-std::unique_ptr<CPDF_Object> CPDF_Null::Clone() const {
-  return pdfium::MakeUnique<CPDF_Null>();
+CFWL_MessageType CFWL_MsgKey::GetClassID() const {
+  return CFWL_MessageType::Key;
 }