Move DOMString back to script

This entirely removes the 'non-geckolib' feature of the util crate.

components/script/lib.rs

@@ -14,6 +14,7 @@
 #![feature(mpsc_select)]
 #![feature(nonzero)]
 #![feature(on_unimplemented)]
+#![feature(optin_builtin_traits)]
 #![feature(peekable_is_empty)]
 #![feature(plugin)]
 #![feature(slice_patterns)]