Move the message definitions to their own files.

xfa/fwl/core/cfwl_msgmouse.cpp

Index: xfa/fwl/core/cfwl_msgmouse.cpp
diff --git a/xfa/fwl/core/cfwl_msgmouse.cpp b/xfa/fwl/core/cfwl_msgmouse.cpp
new file mode 100644
index 0000000000000000000000000000000000000000..ff28cdc9c81aae99a790e5351c51e8277df17080
--- /dev/null
+++ b/xfa/fwl/core/cfwl_msgmouse.cpp
@@ -0,0 +1,21 @@
+// 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_msgmouse.h"
+
+#include "third_party/base/ptr_util.h"
+
+CFWL_MsgMouse::CFWL_MsgMouse() {}
+
+CFWL_MsgMouse::~CFWL_MsgMouse() {}
+
+std::unique_ptr<CFWL_Message> CFWL_MsgMouse::Clone() {
+  return pdfium::MakeUnique<CFWL_MsgMouse>(*this);
+}
+
+CFWL_MessageType CFWL_MsgMouse::GetClassID() const {
+  return CFWL_MessageType::Mouse;
+}