LibWeb: Implement HTMLOrSVGElement.tabIndex

Author: https://github.com/Lubrsi
Commit: 1473bc9169
Pull-request: https://github.com/SerenityOS/serenity/pull/15964
Reviewed-by: https://github.com/awesomekling

Userland/Libraries/LibWeb/HTML/HTMLIFrameElement.h

@@ -28,9 +28,11 @@ public:
 private:
     HTMLIFrameElement(DOM::Document&, DOM::QualifiedName);
 
+    // ^DOM::Element
     virtual void inserted() override;
     virtual void removed_from(Node*) override;
     virtual void parse_attribute(FlyString const& name, String const& value) override;
+    virtual i32 default_tab_index_value() const override;
 
     // https://html.spec.whatwg.org/multipage/iframe-embed-object.html#process-the-iframe-attributes
     void process_the_iframe_attributes(bool initial_insertion = false);