LibWeb: Avoid dynamic_cast in EventTarget initialization

Let's use a virtual to tell if something uses the
WindowOrWorkerGlobalScope mixin instead.

Libraries/LibWeb/DOM/EventTarget.cpp

@@ -67,7 +67,7 @@ void EventTarget::initialize(JS::Realm& realm)
 
     // FIXME: We can't do this for HTML::Window or HTML::WorkerGlobalScope, as this will run when creating the initial global object.
     //        During this time, the ESO is not setup, so it will cause a nullptr dereference in host_defined_intrinsics.
-    if (!is<HTML::WindowOrWorkerGlobalScopeMixin>(this))
+    if (!is_window_or_worker_global_scope_mixin())
         WEB_SET_PROTOTYPE_FOR_INTERFACE(EventTarget);
 }
 

Libraries/LibWeb/DOM/EventTarget.h

@@ -59,6 +59,8 @@ public:
     bool has_event_listener(FlyString const& type) const;
     bool has_event_listeners() const;
 
+    virtual bool is_window_or_worker_global_scope_mixin() const { return false; }
+
 protected:
     explicit EventTarget(JS::Realm&, MayInterfereWithIndexedPropertyAccess = MayInterfereWithIndexedPropertyAccess::No);
 

Libraries/LibWeb/HTML/Window.h

@@ -269,6 +269,8 @@ public:
 private:
     explicit Window(JS::Realm&);
 
+    virtual bool is_window_or_worker_global_scope_mixin() const final { return true; }
+
     virtual void visit_edges(Cell::Visitor&) override;
     virtual void finalize() override;
 

Libraries/LibWeb/HTML/WorkerGlobalScope.h

@@ -112,6 +112,8 @@ protected:
     GC::Ptr<MessagePort> m_internal_port;
 
 private:
+    virtual bool is_window_or_worker_global_scope_mixin() const final { return true; }
+
     virtual void visit_edges(Cell::Visitor&) override;
 
     GC::Ptr<WorkerLocation> m_location;