LibWeb: Add a to_element function to ARIAMixin

This change adds a virtual to_element function to ARIAMixin, and
overrides it in DOM::Element so it can then be used back inside
ARIAMixin to get an element when needed (for example, when computing a
role requires checking the roles of ancestors of an element).

Libraries/LibWeb/ARIA/ARIAMixin.cpp

@@ -6,6 +6,7 @@
 
 #include <LibWeb/ARIA/ARIAMixin.h>
 #include <LibWeb/ARIA/Roles.h>
+#include <LibWeb/DOM/Element.h>
 #include <LibWeb/Infra/CharacterTypes.h>
 
 namespace Web::ARIA {

Libraries/LibWeb/ARIA/ARIAMixin.h

@@ -28,6 +28,8 @@ public:
     // https://www.w3.org/TR/html-aria/#docconformance
     virtual Optional<Role> default_role() const { return {}; }
 
+    virtual DOM::Element const* to_element() const { return {}; }
+
     Optional<Role> role_from_role_attribute_value() const;
     Optional<Role> role_or_default() const;
 

Libraries/LibWeb/DOM/Element.h

@@ -315,6 +315,8 @@ public:
 
     virtual bool include_in_accessibility_tree() const override;
 
+    virtual Element const* to_element() const override { return this; }
+
     bool is_hidden() const;
     bool has_hidden_ancestor() const;