Make down-conversion explicit from CFX_Widetring to CFX_WideStringC.

xfa/fxfa/parser/xfa_document_serialize.cpp

Index: xfa/fxfa/parser/xfa_document_serialize.cpp
diff --git a/xfa/fxfa/parser/xfa_document_serialize.cpp b/xfa/fxfa/parser/xfa_document_serialize.cpp
index 448acbdd1db1bd6a4c066c3221fce360e2ea1cc5..100e54793f418803df5869e6378140348451a6de 100644
--- a/xfa/fxfa/parser/xfa_document_serialize.cpp
+++ b/xfa/fxfa/parser/xfa_document_serialize.cpp
@@ -297,7 +297,7 @@ static void XFA_DataExporter_RegenerateFormFile_Changed(
         buf << FX_WSTRC(L" xmlns=\"\"\n>");
         for (int32_t i = 0; i < wsSelTextArray.GetSize(); i++) {
           buf << FX_WSTRC(L"<value\n>");
-          buf << XFA_ExportEncodeContent(wsSelTextArray[i]);
+          buf << XFA_ExportEncodeContent(wsSelTextArray[i].AsWideStringC());
           buf << FX_WSTRC(L"</value\n>");
         }
         buf << FX_WSTRC(L"</");