LibWeb: Implement caching of reflected element array attributes

For attributes like Element.ariaControlsElements, which are a reflection
of FrozenArray<Element>, we must return the same JS::Array object every
time the attribute is invoked - until its contents have changed. This
patch implements caching of the reflected array in accordance with the
spec.

Libraries/LibWeb/DOM/Element.cpp

@@ -106,6 +106,7 @@ void Element::visit_edges(Cell::Visitor& visitor)
     Base::visit_edges(visitor);
     SlottableMixin::visit_edges(visitor);
     Animatable::visit_edges(visitor);
+    ARIAMixin::visit_edges(visitor);
 
     visitor.visit(m_attributes);
     visitor.visit(m_inline_style);