LibJS+LibWeb: Use new Cell::Visitor helpers to avoid manual iteration

Author: https://github.com/awesomekling
Commit: 53d0dd4a2e
Pull-request: https://github.com/SerenityOS/serenity/pull/23971
Reviewed-by: https://github.com/trflynn89

Userland/Libraries/LibWeb/WebIDL/Promise.cpp

@@ -192,9 +192,7 @@ struct WaitForAllResults : JS::Cell {
     {
         Base::visit_edges(visitor);
         visitor.visit(success_steps);
-        for (auto& value : result) {
-            visitor.visit(value);
-        }
+        visitor.visit(result);
     }
 
     JS::NonnullGCPtr<JS::HeapFunction<void(Vector<JS::Value> const&)>> success_steps;