Bug 1912083 - Adjust Thunderbird code for "Switch url argument to UTF8 string.". rs=bustage-fix

Port https://hg.mozilla.org/mozilla-central/rev/245988a0e81b

mailnews/base/src/nsMsgProgress.cpp

@@ -66,7 +66,7 @@ NS_IMETHODIMP nsMsgProgress::OpenProgressDialog(
   nsString chromeOptions(u"chrome,dependent,centerscreen"_ns);
   if (inDisplayModal) chromeOptions.AppendLiteral(",modal");
 
-  return parent->OpenDialog(NS_ConvertASCIItoUTF16(dialogURL), u"_blank"_ns,
+  return parent->OpenDialog(nsDependentCString(dialogURL), u"_blank"_ns,
                             chromeOptions, array, getter_AddRefs(newWindow));
 }