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/HTMLBRElement.h

@@ -18,6 +18,7 @@ public:
     virtual ~HTMLBRElement() override;
 
     virtual JS::GCPtr<Layout::Node> create_layout_node(CSS::StyleProperties) override;
+    virtual void adjust_computed_style(CSS::StyleProperties&) override;
 
 private:
     virtual bool is_html_br_element() const override { return true; }