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/HTMLFrameElement.h

@@ -28,6 +28,7 @@ private:
     virtual void removed_from(Node*) override;
     virtual void attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value) override;
     virtual i32 default_tab_index_value() const override;
+    virtual void adjust_computed_style(CSS::StyleProperties&) override;
 
     void process_the_frame_attributes(bool initial_insertion = false);
 };