Auto merge of #12374 - jdm:jsup, r=Manishearth

Improve performance of HTMLDivElement constructor

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

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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)
     }
 }