Add type cast definitions for CPDF_Stream.

core/src/fpdfapi/fpdf_parser/fpdf_parser_document.cpp

 // 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
 
 #include "../../../include/fpdfapi/fpdf_parser.h"
 #include "../../../include/fpdfapi/fpdf_module.h"
 
 
@@ skipping 320 matching lines @@
 FX_BOOL CPDF_Document::IsOwner() const {
   if (m_pParser == NULL) {
     return TRUE;
   }
   return m_pParser->IsOwner();
 }
 FX_BOOL CPDF_Document::IsFormStream(FX_DWORD objnum, FX_BOOL& bForm) const {
   {
     CPDF_Object* pObj;
     if (m_IndirectObjs.Lookup((void*)(uintptr_t)objnum, (void*&)pObj)) {
-      bForm = pObj->GetType() == PDFOBJ_STREAM &&
-              ((CPDF_Stream*)pObj)->GetDict()->GetString(FX_BSTRC("Subtype")) ==
+      CPDF_Stream* pStream = pObj->AsStream();
+      bForm = pStream &&
+              pStream->GetDict()->GetString(FX_BSTRC("Subtype")) ==
                   FX_BSTRC("Form");
       return TRUE;
     }
   }
   if (m_pParser == NULL) {
     bForm = FALSE;
     return TRUE;
   }
   return m_pParser->IsFormStream(objnum, bForm);
 }
 void CPDF_Document::ClearPageData() {
   if (m_pDocPage) {
     CPDF_ModuleMgr::Get()->GetPageModule()->ClearDoc(this);
   }
 }
 void CPDF_Document::ClearRenderData() {
   if (m_pDocRender) {
     CPDF_ModuleMgr::Get()->GetRenderModule()->ClearDocData(m_pDocRender);
   }
 }