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/bindings/interface.rs

@@ -226,7 +226,7 @@ pub unsafe fn create_global_object(
     // avoid getting trace hooks called on a partially initialized object.
     JS_SetReservedSlot(rval.get(), DOM_OBJECT_SLOT, PrivateValue(private));
     let proto_array: Box<ProtoOrIfaceArray> =
-        box [0 as *mut JSObject; PrototypeList::PROTO_OR_IFACE_LENGTH];
+        Box::new([0 as *mut JSObject; PrototypeList::PROTO_OR_IFACE_LENGTH]);
     JS_SetReservedSlot(rval.get(),
                        DOM_PROTOTYPE_SLOT,
                        PrivateValue(Box::into_raw(proto_array) as *const libc::c_void));

components/script/dom/bindings/iterable.rs

@@ -62,12 +62,12 @@ impl<T: DomObject + JSTraceable + Iterable> IterableIterator<T> {
                type_: IteratorType,
                wrap: unsafe fn(*mut JSContext, &GlobalScope, Box<IterableIterator<T>>)
                      -> DomRoot<Self>) -> DomRoot<Self> {
-        let iterator = box IterableIterator {
+        let iterator = Box::new(IterableIterator {
             reflector: Reflector::new(),
             type_: type_,
             iterable: Dom::from_ref(iterable),
             index: Cell::new(0),
-        };
+        });
         reflect_dom_object(iterator, &*iterable.global(), wrap)
     }
 

components/script/dom/bindings/trace.rs

@@ -759,7 +759,7 @@ unsafe impl<T: JSTraceable + 'static> JSTraceable for RootedTraceableBox<T> {
 impl<T: JSTraceable + 'static> RootedTraceableBox<T> {
     /// DomRoot a JSTraceable thing for the life of this RootedTraceable
     pub fn new(traceable: T) -> RootedTraceableBox<T> {
-        let traceable = Box::into_raw(box traceable);
+        let traceable = Box::into_raw(Box::new(traceable));
         unsafe {
             RootedTraceableSet::add(traceable);
         }

components/script/dom/bindings/weakref.rs

@@ -56,10 +56,10 @@ pub trait WeakReferenceable: DomObject + Sized {
                               .to_private() as *mut WeakBox<Self>;
             if ptr.is_null() {
                 trace!("Creating new WeakBox holder for {:p}.", self);
-                ptr = Box::into_raw(box WeakBox {
+                ptr = Box::into_raw(Box::new(WeakBox {
                     count: Cell::new(1),
                     value: Cell::new(Some(NonZero::new_unchecked(self))),
-                });
+                }));
                 JS_SetReservedSlot(object, DOM_WEAK_SLOT, PrivateValue(ptr as *const c_void));
             }
             let box_ = &*ptr;