servo: Merge #17259 - Force HTMLStyleElement to create a new CSSStyleSheet when re-parsing (from jyc:new-CSSStyleSheet); r=emilio

- [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 I'm pushing to find out what test results this changes; this is part of a series of PRs to fix bug #17182

Source-Repo: https://github.com/servo/servo
Source-Revision: 7e493529f0eca00b92274a047f843b990e6bd1a9

--HG--
extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear
extra : subtree_revision : cdaab5bc80335f991b90221f00e9bb6acfeb513c

servo/components/script/dom/htmlstyleelement.rs

@@ -111,6 +111,7 @@ impl HTMLStyleElement {
 
         win.layout_chan().send(Msg::AddStylesheet(sheet.clone())).unwrap();
         *self.stylesheet.borrow_mut() = Some(sheet);
+        self.cssom_stylesheet.set(None);
         doc.invalidate_stylesheets();
     }