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/htmlparagraphelement.rs b/components/script/dom/htmlparagraphelement.rs
index f17b7653b6f..bcebc146daf 100644
--- a/components/script/dom/htmlparagraphelement.rs
+++ b/components/script/dom/htmlparagraphelement.rs
@@ -29,7 +29,7 @@ impl HTMLParagraphElement {
     pub fn new(local_name: LocalName,
                prefix: Option<Prefix>,
                document: &Document) -> DomRoot<HTMLParagraphElement> {
-        Node::reflect_node(box HTMLParagraphElement::new_inherited(local_name, prefix, document),
+        Node::reflect_node(Box::new(HTMLParagraphElement::new_inherited(local_name, prefix, document)),
                            document,
                            HTMLParagraphElementBinding::Wrap)
     }