Make down-conversion explicit from CFX_ByteString to CFX_ByteStringC.

core/fpdfdoc/cpvt_fontmap.cpp

Index: core/fpdfdoc/cpvt_fontmap.cpp
diff --git a/core/fpdfdoc/cpvt_fontmap.cpp b/core/fpdfdoc/cpvt_fontmap.cpp
index 529f85169e569e37d90a90f5983aa656df8de8c7..0aa7ac3486da57bc63acc82642f44c2627fe3830 100644
--- a/core/fpdfdoc/cpvt_fontmap.cpp
+++ b/core/fpdfdoc/cpvt_fontmap.cpp
@@ -38,8 +38,9 @@ void CPVT_FontMap::GetAnnotSysPDFFont(CPDF_Document* pDoc,
     return;
 
   if (CPDF_Dictionary* pFontList = pResDict->GetDictBy("Font")) {
-    if (!pFontList->KeyExist(sSysFontAlias))
-      pFontList->SetAtReference(sSysFontAlias, pDoc, pPDFFont->GetFontDict());
+    if (!pFontList->KeyExist(sSysFontAlias.AsByteStringC()))
+      pFontList->SetAtReference(sSysFontAlias.AsByteStringC(), pDoc,
+                                pPDFFont->GetFontDict());
   }
   pSysFont = pPDFFont;
 }