Force HTMLStyleElement to create a new CSSStyleSheet when re-parsing.

We already call Document::invalidate_style_sheets and set
the stylesheet member to a new Stylesheet. This matches the behavior of
Firefox, and means the new CSSStyleSheet you get from accessing .sheet
from JS will be correct instead of stale.

(::get_cssom_stylesheet already tries to use the new Stylesheet, but
MutNullableJS::or_init is called, so if we already created a
CSSStyleSheet we will continue to return that one).

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();
     }