Make CPDF_Object::GetString() a virtual method.

core/src/fpdfdoc/doc_formfield.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/fpdfdoc/fpdf_doc.h"
 #include "doc_utils.h"
 
 FX_BOOL PDF_FormField_IsUnison(CPDF_FormField *pField)
@@ skipping 1066 matching lines @@
     m_pForm->m_bUpdated = TRUE;
     return TRUE;
 }
 void CPDF_FormField::LoadDA()
 {
     CFX_ByteString DA;
     if (CPDF_Object* pObj_t = FPDF_GetFieldAttr(m_pDict, "DA")) {
         DA = pObj_t->GetString();
     }
     if (DA.IsEmpty() && m_pForm->m_pFormDict) {
-        DA = m_pForm->m_pFormDict->GetString("DA");
+        DA = m_pForm->m_pFormDict->GetStringAt("DA");
     }
     if (DA.IsEmpty()) {
         return;
     }
     CPDF_SimpleParser syntax(DA);
     syntax.FindTagParam("Tf", 2);
     CFX_ByteString font_name = syntax.GetWord();
     CPDF_Dictionary* pFontDict = NULL;
     if (m_pForm->m_pFormDict && m_pForm->m_pFormDict->GetDict("DR") &&
         m_pForm->m_pFormDict->GetDict("DR")->GetDict("Font") )
     pFontDict = m_pForm->m_pFormDict->GetDict("DR")->GetDict("Font")->GetDict(font_name);
 
     if (pFontDict == NULL) {
         return;
     }
     m_pFont = m_pForm->m_pDocument->LoadFont(pFontDict);
     m_FontSize = FX_atof(syntax.GetWord());
 }