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

@@ -217,9 +217,12 @@ Root
 HTMLDivElement
 >
 {
-let
-element
-=
+Node
+:
+:
+reflect_node
+(
+box
 HTMLDivElement
 :
 :
@@ -229,14 +232,6 @@ localName
 prefix
 document
 )
-;
-Node
-:
-:
-reflect_node
-(
-box
-element
 document
 HTMLDivElementBinding
 :