Move the message definitions to their own files.

xfa/fwl/core/cfwl_msgsetfocus.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_MSGSETFOCUS_H_
+#define XFA_FWL_CORE_CFWL_MSGSETFOCUS_H_
+
+#include "xfa/fwl/core/cfwl_message.h"
+
+class CFWL_MsgSetFocus : public CFWL_Message {
+ public:
+  CFWL_MsgSetFocus();
+  ~CFWL_MsgSetFocus() override;
+
+  // CFWL_Message
+  std::unique_ptr<CFWL_Message> Clone() override;
+  CFWL_MessageType GetClassID() const override;
+
+  IFWL_Widget* m_pKillFocus;
+};
+
+#endif  // XFA_FWL_CORE_CFWL_MSGSETFOCUS_H_