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`).
 ///

components/script/dom/canvasrenderingcontext2d.rs

@@ -41,7 +41,7 @@ impl CanvasRenderingContext2D {
     }
 
     pub fn recreate(&self, size: Size2D<i32>) {
-        self.renderer.send(Recreate(size));
+        self.renderer.deref().send(Recreate(size));
     }
 }
 
@@ -52,17 +52,17 @@ impl<'a> CanvasRenderingContext2DMethods for JSRef<'a, CanvasRenderingContext2D>
 
     fn FillRect(&self, x: f64, y: f64, width: f64, height: f64) {
         let rect = Rect(Point2D(x as f32, y as f32), Size2D(width as f32, height as f32));
-        self.renderer.send(FillRect(rect));
+        self.renderer.deref().send(FillRect(rect));
     }
 
     fn ClearRect(&self, x: f64, y: f64, width: f64, height: f64) {
         let rect = Rect(Point2D(x as f32, y as f32), Size2D(width as f32, height as f32));
-        self.renderer.send(ClearRect(rect));
+        self.renderer.deref().send(ClearRect(rect));
     }
 
     fn StrokeRect(&self, x: f64, y: f64, width: f64, height: f64) {
         let rect = Rect(Point2D(x as f32, y as f32), Size2D(width as f32, height as f32));
-        self.renderer.send(StrokeRect(rect));
+        self.renderer.deref().send(StrokeRect(rect));
     }
 }
 

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 {

components/script/dom/workerglobalscope.rs

@@ -85,7 +85,7 @@ impl<'a> WorkerGlobalScopeMethods for JSRef<'a, WorkerGlobalScope> {
 
     fn Location(&self) -> Temporary<WorkerLocation> {
         if self.location.get().is_none() {
-            let location = WorkerLocation::new(*self, self.worker_url.clone());
+            let location = WorkerLocation::new(*self, self.worker_url.deref().clone());
             self.location.assign(Some(location));
         }
         Temporary::new(self.location.get().get_ref().clone())
@@ -110,7 +110,7 @@ impl<'a> WorkerGlobalScopeMethods for JSRef<'a, WorkerGlobalScope> {
                 }
             };
 
-            match self.js_context.evaluate_script(
+            match self.js_context.deref().evaluate_script(
                 self.reflector().get_jsobject(), source, url.serialize(), 1) {
                 Ok(_) => (),
                 Err(_) => {

components/script/dom/workerlocation.rs

@@ -38,7 +38,7 @@ impl WorkerLocation {
 
 impl<'a> WorkerLocationMethods for JSRef<'a, WorkerLocation> {
     fn Href(&self) -> DOMString {
-        self.url.serialize()
+        self.url.deref().serialize()
     }
 
     fn Search(&self) -> DOMString {