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/htmlformelement.rs b/components/script/dom/htmlformelement.rs
index b58194bed62..d1731e6f2ff 100755
--- a/components/script/dom/htmlformelement.rs
+++ b/components/script/dom/htmlformelement.rs
@@ -86,7 +86,7 @@ impl HTMLFormElement {
     pub fn new(local_name: LocalName,
                prefix: Option<Prefix>,
                document: &Document) -> DomRoot<HTMLFormElement> {
-        Node::reflect_node(box HTMLFormElement::new_inherited(local_name, prefix, document),
+        Node::reflect_node(Box::new(HTMLFormElement::new_inherited(local_name, prefix, document)),
                            document,
                            HTMLFormElementBinding::Wrap)
     }
@@ -217,7 +217,7 @@ impl HTMLFormElementMethods for HTMLFormElement {
             }
         }
         DomRoot::from_ref(self.elements.init_once(|| {
-            let filter = box ElementsFilter { form: DomRoot::from_ref(self) };
+            let filter = Box::new(ElementsFilter { form: DomRoot::from_ref(self) });
             let window = window_from_node(self);
             HTMLFormControlsCollection::new(&window, self.upcast(), filter)
         }))