Fix IWYU in formfiller/ directory.

fpdfsdk/include/formfiller/FFL_ComboBox.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 _FFL_COMBOBOX_H_
- #define _FFL_COMBOBOX_H_
+#define _FFL_COMBOBOX_H_
+
+#include "../../../core/include/fxcrt/fx_string.h"
+#include "FFL_FormFiller.h"
+
+class CBA_FontMap;
+class CPDFSDK_Document;
 
 struct FFL_ComboBoxState
 {
         int nIndex;
         int nStart;
         int nEnd;
         CFX_WideString sValue;
 };
-class CBA_FontMap;
 
 class CFFL_ComboBox : public CFFL_FormFiller, public IPWL_FocusHandler, public IPWL_Edit_Notify
 {
 public:
         CFFL_ComboBox(CPDFDoc_Environment* pApp, CPDFSDK_Annot* pWidget);
         virtual ~CFFL_ComboBox();
         
         virtual PWL_CREATEPARAM         GetCreateParam();
         virtual CPWL_Wnd*                       NewPDFWindow(const PWL_CREATEPARAM& cp, CPDFSDK_PageView* pPageView);
 
@@ skipping 32 matching lines @@
         CFX_WideString                          GetSelectExportText();
 
 private:
         CBA_FontMap*                            m_pFontMap;
         FFL_ComboBoxState                               m_State;
         //CFFL_IM_BOX                                   m_IMBox;
 };
 
 #endif //_FFL_COMBOBOX_H_