servo: Merge #12374 - Improve performance of HTMLDivElement constructor (from jdm:jsup); r=Manishearth

These changes address two sources of performance loss seen while profiling in #12354. #12358 and rust-lang/rust#34727 are still the biggest offenders, however.

---
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes do not require tests because we don't have performance tests and these are only optimizations

Source-Repo: https://github.com/servo/servo
Source-Revision: 01ec8491d3200d6710336da1bb0f4e01b49dc4bc

UltraBlame original commit: 383b98548e4b848e35322e195dff638072fa2da0

Author: marco-c

servo/components/script/dom/htmldivelement.rs

@@ -28,8 +28,9 @@ impl HTMLDivElement {
     pub fn new(localName: Atom,
                prefix: Option<DOMString>,
                document: &Document) -> Root<HTMLDivElement> {
-        let element = HTMLDivElement::new_inherited(localName, prefix, document);
-        Node::reflect_node(box element, document, HTMLDivElementBinding::Wrap)
+        Node::reflect_node(box HTMLDivElement::new_inherited(localName, prefix, document),
+                           document,
+                           HTMLDivElementBinding::Wrap)
     }
 }