LibWeb: Compute display: contents as none for unusual elements

https://drafts.csswg.org/css-display-3/#unbox-html specifies certain
elements that have their `display` style computed as `none` when
specified as `contents`.

This fixes at least one WPT test:
http://wpt.live/css/css-display/display-contents-suppression-dynamic-001.html

Libraries/LibWeb/HTML/HTMLEmbedElement.h

@@ -23,6 +23,7 @@ private:
     virtual bool is_html_embed_element() const override { return true; }
     virtual void initialize(JS::Realm&) override;
     virtual void apply_presentational_hints(CSS::StyleProperties&) const override;
+    virtual void adjust_computed_style(CSS::StyleProperties&) override;
 };
 
 }