Make CPDF_Object::GetString() a virtual method.

core/src/fpdfdoc/doc_ap.cpp

Index: core/src/fpdfdoc/doc_ap.cpp
diff --git a/core/src/fpdfdoc/doc_ap.cpp b/core/src/fpdfdoc/doc_ap.cpp
index 2d991b39c2b17403279a2e355028fea7d40f4e0e..2b4457951c6c9af996852145ad8b2538ad3f28e2 100644
--- a/core/src/fpdfdoc/doc_ap.cpp
+++ b/core/src/fpdfdoc/doc_ap.cpp
@@ -28,7 +28,8 @@ FX_BOOL FPDF_GenerateAP(CPDF_Document* pDoc, CPDF_Dictionary* pAnnotDict)
             if (!pAnnotDict->KeyExist("AS")) {
                 if (CPDF_Dictionary* pParentDict = pAnnotDict->GetDict("Parent")) {
                     if (pParentDict->KeyExist("AS")) {
-                        pAnnotDict->SetAtString("AS", pParentDict->GetString("AS"));
+                        pAnnotDict->SetAtString(
+                            "AS", pParentDict->GetStringAt("AS"));
                     }
                 }
             }
@@ -258,7 +259,7 @@ static FX_BOOL GenerateWidgetAP(CPDF_Document* pDoc, CPDF_Dictionary* pAnnotDict
         DA = pDAObj->GetString();
     }
     if (DA.IsEmpty()) {
-        DA = pFormDict->GetString("DA");
+        DA = pFormDict->GetStringAt("DA");
     }
     if (DA.IsEmpty()) {
         return FALSE;
@@ -344,7 +345,7 @@ static FX_BOOL GenerateWidgetAP(CPDF_Document* pDoc, CPDF_Dictionary* pAnnotDict
         if (CPDF_Array * pArray = pBSDict->GetArray("D")) {
             dsBorder = CPVT_Dash(pArray->GetInteger(0), pArray->GetInteger(1), pArray->GetInteger(2));
         }
-        switch (pBSDict->GetString("S").GetAt(0)) {
+        switch (pBSDict->GetStringAt("S").GetAt(0)) {
             case 'S':
                 nBorderStyle = PBS_SOLID;
                 break;