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, 12 insertions, 8 deletions

diff --git a/components/script/dom/attr.rs b/components/script/dom/attr.rs
index 10e0ee6bb5c..c50ccae3e8e 100644
--- a/components/script/dom/attr.rs
+++ b/components/script/dom/attr.rs
@@ -64,14 +64,18 @@ impl Attr {
                prefix: Option<Prefix>,
                owner: Option<&Element>)
                -> DomRoot<Attr> {
-        reflect_dom_object(box Attr::new_inherited(local_name,
-                                                   value,
-                                                   name,
-                                                   namespace,
-                                                   prefix,
-                                                   owner),
-                           window,
-                           AttrBinding::Wrap)
+        reflect_dom_object(
+            Box::new(Attr::new_inherited(
+                local_name,
+                value,
+                name,
+                namespace,
+                prefix,
+                owner
+            )),
+            window,
+            AttrBinding::Wrap
+        )
     }
 
     #[inline]