Remove use of unstable box syntax.

http://www.robohornet.org gives a score of 101.36 on master,
and 102.68 with this PR. The latter is slightly better,
but probably within noise level.
So it looks like this PR does not affect DOM performance.

This is expected since `Box::new` is defined as:

```rust
impl<T> Box<T> {
    #[inline(always)]
    pub fn new(x: T) -> Box<T> {
        box x
    }
}
```

With inlining, it should compile to the same as box syntax.

components/script/dom/dedicatedworkerglobalscope.rs

@@ -133,16 +133,18 @@ impl DedicatedWorkerGlobalScope {
                closing: Arc<AtomicBool>)
                -> DomRoot<DedicatedWorkerGlobalScope> {
         let cx = runtime.cx();
-        let scope = box DedicatedWorkerGlobalScope::new_inherited(init,
-                                                                  worker_url,
-                                                                  from_devtools_receiver,
-                                                                  runtime,
-                                                                  parent_sender,
-                                                                  own_sender,
-                                                                  receiver,
-                                                                  timer_event_chan,
-                                                                  timer_event_port,
-                                                                  closing);
+        let scope = Box::new(DedicatedWorkerGlobalScope::new_inherited(
+            init,
+            worker_url,
+            from_devtools_receiver,
+            runtime,
+            parent_sender,
+            own_sender,
+            receiver,
+            timer_event_chan,
+            timer_event_port,
+            closing
+        ));
         unsafe {
             DedicatedWorkerGlobalScopeBinding::Wrap(cx, scope)
         }
@@ -193,8 +195,10 @@ impl DedicatedWorkerGlobalScope {
                                                               &init.resource_threads.sender()) {
                 Err(_) => {
                     println!("error loading script {}", serialized_worker_url);
-                    parent_sender.send(CommonScriptMsg::Task(WorkerEvent,
-                        box SimpleWorkerErrorHandler::new(worker))).unwrap();
+                    parent_sender.send(CommonScriptMsg::Task(
+                        WorkerEvent,
+                        Box::new(SimpleWorkerErrorHandler::new(worker))
+                    )).unwrap();
                     return;
                 }
                 Ok((metadata, bytes)) => (metadata, bytes)
@@ -211,10 +215,10 @@ impl DedicatedWorkerGlobalScope {
             let (timer_tx, timer_rx) = channel();
             let (timer_ipc_chan, timer_ipc_port) = ipc::channel().unwrap();
             let worker_for_route = worker.clone();
-            ROUTER.add_route(timer_ipc_port.to_opaque(), box move |message| {
+            ROUTER.add_route(timer_ipc_port.to_opaque(), Box::new(move |message| {
                 let event = message.to().unwrap();
                 timer_tx.send((worker_for_route.clone(), event)).unwrap();
-            });
+            }));
 
             let global = DedicatedWorkerGlobalScope::new(
                 init, url, devtools_mpsc_port, runtime,
@@ -257,19 +261,19 @@ impl DedicatedWorkerGlobalScope {
     }
 
     pub fn script_chan(&self) -> Box<ScriptChan + Send> {
-        box WorkerThreadWorkerChan {
+        Box::new(WorkerThreadWorkerChan {
             sender: self.own_sender.clone(),
             worker: self.worker.borrow().as_ref().unwrap().clone(),
-        }
+        })
     }
 
     pub fn new_script_pair(&self) -> (Box<ScriptChan + Send>, Box<ScriptPort + Send>) {
         let (tx, rx) = channel();
-        let chan = box SendableWorkerScriptChan {
+        let chan = Box::new(SendableWorkerScriptChan {
             sender: tx,
             worker: self.worker.borrow().as_ref().unwrap().clone(),
-        };
-        (chan, box rx)
+        });
+        (chan, Box::new(rx))
     }
 
     #[allow(unsafe_code)]
@@ -355,7 +359,7 @@ impl DedicatedWorkerGlobalScope {
     #[allow(unsafe_code)]
     pub fn forward_error_to_worker_object(&self, error_info: ErrorInfo) {
         let worker = self.worker.borrow().as_ref().unwrap().clone();
-        let task = box task!(forward_error_to_worker_object: move || {
+        let task = Box::new(task!(forward_error_to_worker_object: move || {
             let worker = worker.root();
             let global = worker.global();
 
@@ -378,7 +382,7 @@ impl DedicatedWorkerGlobalScope {
             if event_status == EventStatus::NotCanceled {
                 global.report_an_error(error_info, unsafe { NullHandleValue });
             }
-        });
+        }));
         // TODO: Should use the DOM manipulation task source.
         self.parent_sender.send(CommonScriptMsg::Task(WorkerEvent, task)).unwrap();
     }
@@ -401,9 +405,9 @@ impl DedicatedWorkerGlobalScopeMethods for DedicatedWorkerGlobalScope {
     unsafe fn PostMessage(&self, cx: *mut JSContext, message: HandleValue) -> ErrorResult {
         let data = StructuredCloneData::write(cx, message)?;
         let worker = self.worker.borrow().as_ref().unwrap().clone();
-        let task = box task!(post_worker_message: move || {
+        let task = Box::new(task!(post_worker_message: move || {
             Worker::handle_message(worker, data);
-        });
+        }));
         self.parent_sender.send(CommonScriptMsg::Task(WorkerEvent, task)).unwrap();
         Ok(())
     }