LibWeb: Add fast_is<T>() for HTML::HTMLHtmlElement

Another one spotted in a scroll-up-and-down profile.

Userland/Libraries/LibWeb/HTML/HTMLHtmlElement.h

@@ -18,6 +18,14 @@ public:
     virtual ~HTMLHtmlElement() override;
 
     bool should_use_body_background_properties() const;
+
+private:
+    virtual bool is_html_html_element() const override { return true; }
 };
 
 }
+
+namespace Web::DOM {
+template<>
+inline bool Node::fast_is<HTML::HTMLHtmlElement>() const { return is_html_html_element(); }
+}