Add type cast definitions for CPDF_Stream.

core/src/fpdfapi/fpdf_parser/fpdf_parser_document.cpp

Index: core/src/fpdfapi/fpdf_parser/fpdf_parser_document.cpp
diff --git a/core/src/fpdfapi/fpdf_parser/fpdf_parser_document.cpp b/core/src/fpdfapi/fpdf_parser/fpdf_parser_document.cpp
index 23962612763c6564dd81d1c547cf0fb159c4e9ab..5333fe808786f231ee62be815ee7773cda799baa 100644
--- a/core/src/fpdfapi/fpdf_parser/fpdf_parser_document.cpp
+++ b/core/src/fpdfapi/fpdf_parser/fpdf_parser_document.cpp
@@ -338,8 +338,9 @@ 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;
     }