bug 77300, r=ftang, rs=bstell (see Brendan's comments in the bug report)

alias "646" and "ANSI_X3.4-1968" to ascii

intl/uconv/src/charsetalias.properties

@@ -40,6 +40,8 @@
 ##    (e.g. XLFD charset- such as JIS x0201, JIS x0208), don't put here
 ##
 us-ascii=us-ascii
+ansi_x3.4-1968=us-ascii
+646=us-ascii
 iso-8859-1=ISO-8859-1
 iso-8859-2=ISO-8859-2
 iso-8859-3=ISO-8859-3