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.

1 files changed, 1 insertions, 1 deletions

diff --git a/components/script/dom/beforeunloadevent.rs b/components/script/dom/beforeunloadevent.rs
index ad1f3ee2a21..bd7fc05daac 100644
--- a/components/script/dom/beforeunloadevent.rs
+++ b/components/script/dom/beforeunloadevent.rs
@@ -33,7 +33,7 @@ impl BeforeUnloadEvent {
     }
 
     pub fn new_uninitialized(window: &Window) -> DomRoot<BeforeUnloadEvent> {
-        reflect_dom_object(box BeforeUnloadEvent::new_inherited(),
+        reflect_dom_object(Box::new(BeforeUnloadEvent::new_inherited()),
                            window,
                            BeforeUnloadEventBinding::Wrap)
     }