Make CPDF_Object::GetString() a virtual method.

fpdfsdk/src/fpdf_flatten.cpp

Index: fpdfsdk/src/fpdf_flatten.cpp
diff --git a/fpdfsdk/src/fpdf_flatten.cpp b/fpdfsdk/src/fpdf_flatten.cpp
index d3aa8c5ec11c213631a5ae6bcb49d55d203abb0b..663ae9d572255a71f5ad9b529b54b0c2383acdbe 100644
--- a/fpdfsdk/src/fpdf_flatten.cpp
+++ b/fpdfsdk/src/fpdf_flatten.cpp
@@ -102,7 +102,7 @@ int ParserAnnots( CPDF_Document* pSourceDoc, CPDF_Dictionary * pPageDic, CPDF_Re
             continue;
 
         CPDF_Dictionary* pAnnotDic = (CPDF_Dictionary*)pObj;
-        CFX_ByteString sSubtype = pAnnotDic->GetString("Subtype");
+        CFX_ByteString sSubtype = pAnnotDic->GetStringAt("Subtype");
         if (sSubtype == "Popup")
             continue;
 
@@ -445,7 +445,7 @@ DLLEXPORT int STDCALL FPDFPage_Flatten( FPDF_PAGE page, int nFlag)
 		CPDF_Rect rcAnnot = pAnnotDic->GetRect("Rect");
 		rcAnnot.Normalize();
 
-		CFX_ByteString sAnnotState = pAnnotDic->GetString("AS");
+		CFX_ByteString sAnnotState = pAnnotDic->GetStringAt("AS");
 		CPDF_Dictionary* pAnnotAP = pAnnotDic->GetDict("AP");
 		if (!pAnnotAP)continue;