servo: Merge #16170 - Implement HTMLAnchorElement.rel getter and setter (from TheKK:implement_html_anchor_element_rel); r=jdm

This PR makes code below possible:

```javascript
a = document.createElement("a");
a.rel = "foo";
console.log(a.rel); // print out "foo"
```
---
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] There are tests for these changes

Source-Repo: https://github.com/servo/servo
Source-Revision: 2e743c2c56d7841ed9135e72581bf7db5e1092b5

--HG--
extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear
extra : subtree_revision : 0968bc1bd94b382611b548a7243b6831901722f9

servo/components/script/dom/htmlanchorelement.rs

@@ -113,11 +113,17 @@ impl HTMLAnchorElementMethods for HTMLAnchorElement {
         self.upcast::<Node>().SetTextContent(Some(value))
     }
 
+    // https://html.spec.whatwg.org/multipage/#dom-a-rel
+    make_getter!(Rel, "rel");
+
+    // https://html.spec.whatwg.org/multipage/#dom-a-rel
+    fn SetRel(&self, rel: DOMString) {
+        self.upcast::<Element>().set_tokenlist_attribute(&local_name!("rel"), rel);
+    }
+
     // https://html.spec.whatwg.org/multipage/#dom-a-rellist
     fn RelList(&self) -> Root<DOMTokenList> {
-        self.rel_list.or_init(|| {
-            DOMTokenList::new(self.upcast(), &local_name!("rel"))
-        })
+        self.rel_list.or_init(|| DOMTokenList::new(self.upcast(), &local_name!("rel")))
     }
 
     // https://html.spec.whatwg.org/multipage/#dom-a-coords

servo/components/script/dom/webidls/HTMLAnchorElement.webidl

@@ -13,12 +13,12 @@
 // https://html.spec.whatwg.org/multipage/#htmlanchorelement
 interface HTMLAnchorElement : HTMLElement {
   attribute DOMString target;
-  //         attribute DOMString download;
-  //         attribute USVString ping;
-  //         attribute DOMString rel;
+  //       attribute DOMString download;
+  //       attribute USVString ping;
+           attribute DOMString rel;
   readonly attribute DOMTokenList relList;
-  //         attribute DOMString hreflang;
-  //         attribute DOMString type;
+  //       attribute DOMString hreflang;
+  //       attribute DOMString type;
 
   [Pure]
            attribute DOMString text;