Replace Root::deref by a custom get_unsound_ref_forever method.

This will hopefully make it clearer that this is not the correct function
to call.

components/script/dom/urlsearchparams.rs

@@ -51,12 +51,13 @@ impl URLSearchParams {
             },
             Some(eURLSearchParams(u)) => {
                 let u = u.root();
+                let usp = usp.r();
                 let mut map = usp.data.borrow_mut();
                 *map = u.r().data.borrow().clone();
             },
             None => {}
         }
-        Ok(Temporary::from_rooted(*usp))
+        Ok(Temporary::from_rooted(usp.r()))
     }
 }