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/bindings/trace.rs

@@ -132,6 +132,12 @@ impl<T> Deref<T> for Untraceable<T> {
     }
 }
 
+impl<T> DerefMut<T> for Untraceable<T> {
+    fn deref_mut<'a>(&'a mut self) -> &'a mut T {
+        &mut self.inner
+    }
+}
+
 /// Encapsulates a type that can be traced but is boxed in a type we don't
 /// control (such as `RefCell`).
 ///