Split xfa_document_serialize into class files

xfa/fxfa/parser/cxfa_dataimporter.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 XFA_FXFA_PARSER_CXFA_DATAIMPORTER_H_
+#define XFA_FXFA_PARSER_CXFA_DATAIMPORTER_H_
+
+#include "core/fxcrt/include/fx_system.h"
+
+class CXFA_Document;
+class IFX_FileRead;
+
+class CXFA_DataImporter {
+ public:
+  explicit CXFA_DataImporter(CXFA_Document* pDocument);
+
+  FX_BOOL ImportData(IFX_FileRead* pDataDocument);
+
+ protected:
+  CXFA_Document* const m_pDocument;
+};
+
+#endif  // XFA_FXFA_PARSER_CXFA_DATAIMPORTER_H_