Reintroduce Untraceable<T>.deref_mut() to make mem::replace() possible to Untracebale<T> field.

Some compile errors caused by the compiler's misreading comes back again :(
We re-use `deref()`explicitly to hide these errors.

components/script/dom/dedicatedworkerglobalscope.rs

@@ -120,7 +120,7 @@ impl DedicatedWorkerGlobalScope {
             let target: JSRef<EventTarget> =
                 EventTargetCast::from_ref(*global);
             loop {
-                match global.receiver.recv_opt() {
+                match global.receiver.deref().recv_opt() {
                     Ok(DOMMessage(data, nbytes)) => {
                         let mut message = UndefinedValue();
                         unsafe {