Update externs for String.prototype.localeCompare

According to the ECMAScript specification the function String.prototype.localeCompare can deal with optional "locales" and "options" arguments.

See:
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/localeCompare
http://www.ecma-international.org/ecma-402/1.0/#sec-13.1.1
I already signed the CLA a while ago but forgot to put my own name into the CONTRIBUTORS file!