http://lxr.mozilla.org/seamonkey/source/intl/uconv/idl/nsICharsetConverterManager2.idl#109 nsICharsetConverterManager2::GetCharsetAtom takes a string and convert it to atom. The implementaion uses the charset alias manager which is not inexpensive. The function is needed everytime creating decoder/encoder, so we want to improve the performance there. The alias resolution is not needed when the claller knows the standard name like "UTF-8" (case need to match). By adding an option to skip the alias resoultion, we can improve the performance.