LibWeb: Make CustomElementDefinition relationship with GC more sane

1. Stop using GC::Root in member variables, since that usually creates
   a realm leak.

2. Stop putting OrderedHashMap<FlyString, GC::Ptr> on the stack while
   setting these up, since that won't protect the objects from GC.

Libraries/LibWeb/HTML/CustomElements/CustomElementDefinition.cpp

@@ -13,6 +13,11 @@ void CustomElementDefinition::visit_edges(Visitor& visitor)
     Base::visit_edges(visitor);
     visitor.visit(m_constructor);
     visitor.visit(m_lifecycle_callbacks);
+    for (auto& entry : m_construction_stack) {
+        entry.visit(
+            [&](GC::Ref<DOM::Element>& element) { visitor.visit(element); },
+            [&](AlreadyConstructedCustomElementMarker&) {});
+    }
 }
 
 }