Splitting fpdfdoc/doc_* part I

core/fpdfdoc/include/cpdf_actionfields.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 CORE_FPDFDOC_INCLUDE_CPDF_ACTIONFIELDS_H_
 #define CORE_FPDFDOC_INCLUDE_CPDF_ACTIONFIELDS_H_
 
+#include <stddef.h>
+
 #include <vector>
 
 class CPDF_Action;
 class CPDF_Object;
 
 class CPDF_ActionFields {
  public:
   explicit CPDF_ActionFields(const CPDF_Action* pAction) : m_pAction(pAction) {}
 
   size_t GetFieldsCount() const;
   std::vector<CPDF_Object*> GetAllFields() const;
   CPDF_Object* GetField(size_t iIndex) const;
 
  private:
   const CPDF_Action* const m_pAction;
 };
 
 #endif  // CORE_FPDFDOC_INCLUDE_CPDF_ACTIONFIELDS_H_