LibWeb: Convert DOM::AbortSignal to use JS::SafeFunction

This protects the captured GC pointers automatically instead of
manually.

Userland/Libraries/LibWeb/DOM/EventTarget.cpp

@@ -170,9 +170,10 @@ void EventTarget::add_an_event_listener(DOMEventListener& listener)
 
     // 5. If listener’s signal is not null, then add the following abort steps to it:
     if (listener.signal) {
-        listener.signal->add_abort_algorithm([strong_event_target = JS::make_handle(*this), listener = JS::make_handle(&listener)]() mutable {
+        // NOTE: `this` and `listener` are protected by AbortSignal using JS::SafeFunction.
+        listener.signal->add_abort_algorithm([this, &listener]() mutable {
             // 1. Remove an event listener with eventTarget and listener.
-            strong_event_target->remove_an_event_listener(*listener);
+            remove_an_event_listener(listener);
         });
     }
 }