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, 3 insertions, 3 deletions

diff --git a/components/script/dom/htmltableelement.rs b/components/script/dom/htmltableelement.rs
index 2f8e3e3857b..cda13a6496d 100644
--- a/components/script/dom/htmltableelement.rs
+++ b/components/script/dom/htmltableelement.rs
@@ -63,7 +63,7 @@ impl HTMLTableElement {
     #[allow(unrooted_must_root)]
     pub fn new(local_name: LocalName, prefix: Option<Prefix>, document: &Document)
                -> DomRoot<HTMLTableElement> {
-        Node::reflect_node(box HTMLTableElement::new_inherited(local_name, prefix, document),
+        Node::reflect_node(Box::new(HTMLTableElement::new_inherited(local_name, prefix, document)),
                            document,
                            HTMLTableElementBinding::Wrap)
     }
@@ -151,7 +151,7 @@ impl HTMLTableElementMethods for HTMLTableElement {
     // https://html.spec.whatwg.org/multipage/#dom-table-rows
     fn Rows(&self) -> DomRoot<HTMLCollection> {
         let filter = self.get_rows();
-        HTMLCollection::new(&window_from_node(self), self.upcast(), box filter)
+        HTMLCollection::new(&window_from_node(self), self.upcast(), Box::new(filter))
     }
 
     // https://html.spec.whatwg.org/multipage/#dom-table-caption
@@ -264,7 +264,7 @@ impl HTMLTableElementMethods for HTMLTableElement {
 
         self.tbodies.or_init(|| {
             let window = window_from_node(self);
-            let filter = box TBodiesFilter;
+            let filter = Box::new(TBodiesFilter);
             HTMLCollection::create(&window, self.upcast(), filter)
         })
     }