Don't use the Untraceable fields outside the module they're defined in.

This pattern will become illegal with the next Rust upgrade.

src/components/script/dom/location.rs

@@ -18,7 +18,7 @@ use extra::serialize::{Encoder, Encodable};
 #[deriving(Encodable)]
 pub struct Location {
     reflector_: Reflector, //XXXjdm cycle: window->Location->window
-    extra: Untraceable,
+    priv extra: Untraceable,
 }
 
 struct Untraceable {