Move the message definitions to their own files.

xfa/fwl/core/cfwl_msgmousewheel.h

Index: xfa/fwl/core/cfwl_msgmousewheel.h
diff --git a/xfa/fwl/core/cfwl_msgmousewheel.h b/xfa/fwl/core/cfwl_msgmousewheel.h
new file mode 100644
index 0000000000000000000000000000000000000000..c913eae42c8f5204c3525fbcdbebe97eed432d1e
--- /dev/null
+++ b/xfa/fwl/core/cfwl_msgmousewheel.h
@@ -0,0 +1,28 @@
+// 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_MSGMOUSEWHEEL_H_
+#define XFA_FWL_CORE_CFWL_MSGMOUSEWHEEL_H_
+
+#include "xfa/fwl/core/cfwl_message.h"
+
+class CFWL_MsgMouseWheel : public CFWL_Message {
+ public:
+  CFWL_MsgMouseWheel();
+  ~CFWL_MsgMouseWheel() override;
+
+  // CFWL_Message
+  std::unique_ptr<CFWL_Message> Clone() override;
+  CFWL_MessageType GetClassID() const override;
+
+  FX_FLOAT m_fx;
+  FX_FLOAT m_fy;
+  FX_FLOAT m_fDeltaX;
+  FX_FLOAT m_fDeltaY;
+  uint32_t m_dwFlags;
+};
+
+#endif  // XFA_FWL_CORE_CFWL_MSGMOUSEWHEEL_H_