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, 2 insertions, 2 deletions

diff --git a/components/script/dom/bindings/weakref.rs b/components/script/dom/bindings/weakref.rs
index 885fa62cdc1..e3cff6175b5 100644
--- a/components/script/dom/bindings/weakref.rs
+++ b/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;