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/HTMLMeterElement.h

@@ -37,6 +37,8 @@ public:
     virtual void inserted() override;
     virtual void removed_from(DOM::Node*) override;
 
+    virtual void adjust_computed_style(CSS::StyleProperties&) override;
+
     // https://html.spec.whatwg.org/multipage/forms.html#category-label
     virtual bool is_labelable() const override { return true; }