LibWeb: Replace usages of dynamic_cast with as and as_if

Author: https://github.com/tcl3
Commit: aadd563592
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/5953
Reviewed-by: https://github.com/gmta ✅
Reviewed-by: https://github.com/trflynn89

Libraries/LibWeb/HTML/EventLoop/EventLoop.cpp

@@ -596,9 +596,8 @@ void EventLoop::perform_a_microtask_checkpoint()
     // 4. For each environment settings object settingsObject whose responsible event loop is this event loop, notify about rejected promises given settingsObject's global object.
     auto environments = GC::RootVector { heap(), m_related_environment_settings_objects };
     for (auto& environment_settings_object : environments) {
-        auto* global = dynamic_cast<HTML::UniversalGlobalScopeMixin*>(&environment_settings_object->global_object());
-        VERIFY(global);
-        global->notify_about_rejected_promises({});
+        auto& global = as<HTML::UniversalGlobalScopeMixin>(environment_settings_object->global_object());
+        global.notify_about_rejected_promises({});
     }
 
     // 5. Cleanup Indexed Database transactions.