Remove include only fwl.h and IWYU.

xfa/include/fwl/core/fwl_panel.h

 // Copyright 2014 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_INCLUDE_FWL_CORE_FWL_PANEL_H_
 #define XFA_INCLUDE_FWL_CORE_FWL_PANEL_H_
 
-class IFWL_Widget;
+#include "xfa/src/fwl/src/core/include/fwl_widgetimp.h"
+
 class IFWL_Content;
-class IFWL_Panel;
-class IFWL_CustomPanel;
+
 #define FWL_CLASS_Panel L"FWL_Panel"
 #define FWL_CLASSHASH_Panel 881567292
 
 class IFWL_Panel : public IFWL_Widget {
  public:
   static IFWL_Panel* Create(CFWL_WidgetImpProperties& properties,
                             IFWL_Widget* pOuter);
 
   IFWL_Content* GetContent();
   FWL_ERR SetContent(IFWL_Content* pContent);
 
  protected:
   IFWL_Panel();
 };
 
 class IFWL_CustomPanel : public IFWL_Widget {
  public:
   static IFWL_CustomPanel* Create(CFWL_WidgetImpProperties& properties,
                                   IFWL_Widget* pOuter);
 
   IFWL_Content* GetContent();
   FWL_ERR SetContent(IFWL_Content* pContent);
   FWL_ERR SetProxy(IFWL_Proxy* pProxy);
 
  protected:
   IFWL_CustomPanel();
 };
 
 #endif  // XFA_INCLUDE_FWL_CORE_FWL_PANEL_H_