Make CPDF_Object::GetString() a virtual method.

core/src/fpdfdoc/doc_annot.cpp

Index: core/src/fpdfdoc/doc_annot.cpp
diff --git a/core/src/fpdfdoc/doc_annot.cpp b/core/src/fpdfdoc/doc_annot.cpp
index 98cd01ba15c6c233870d62e99cfde66c2eb4133f..4db7e55919a99f9acb114d9ed2d5f8acdbe699de 100644
--- a/core/src/fpdfdoc/doc_annot.cpp
+++ b/core/src/fpdfdoc/doc_annot.cpp
@@ -202,12 +202,12 @@ CPDF_Stream* FPDFDOC_GetAnnotAP(CPDF_Dictionary* pAnnotDict, CPDF_Annot::Appeara
     if (psub->GetType() == PDFOBJ_STREAM) {
         pStream = (CPDF_Stream*)psub;
     } else if (psub->GetType() == PDFOBJ_DICTIONARY) {
-        CFX_ByteString as = pAnnotDict->GetString("AS");
+        CFX_ByteString as = pAnnotDict->GetStringAt("AS");
         if (as.IsEmpty()) {
-            CFX_ByteString value = pAnnotDict->GetString(FX_BSTRC("V"));
+            CFX_ByteString value = pAnnotDict->GetStringAt("V");
             if (value.IsEmpty()) {
                 CPDF_Dictionary* pDict = pAnnotDict->GetDict(FX_BSTRC("Parent"));
-                value = pDict ? pDict->GetString(FX_BSTRC("V")) : CFX_ByteString();
+                value = pDict ? pDict->GetStringAt("V") : CFX_ByteString();
             }
             if (value.IsEmpty() || !((CPDF_Dictionary*)psub)->KeyExist(value)) {
                 as = FX_BSTRC("Off");
@@ -319,7 +319,7 @@ void CPDF_Annot::DrawBorder(CFX_RenderDevice* pDevice, const CFX_AffineMatrix* p
             width = 1;
         }
     } else {
-        CFX_ByteString style = pBS->GetString("S");
+        CFX_ByteString style = pBS->GetStringAt("S");
         pDashArray = pBS->GetArray("D");
         style_char = style[1];
         width = pBS->GetNumber("W");