LibWeb: Move event listeners, handlers and callbacks to the GC heap

This patch moves the following things to being GC-allocated:
- Bindings::CallbackType
- HTML::EventHandler
- DOM::IDLEventListener
- DOM::DOMEventListener
- DOM::NodeFilter

Note that we only use PlatformObject for things that might be exposed
to web content. Anything that is only used internally inherits directly
from JS::Cell instead, making them a bit more lightweight.

Userland/Libraries/LibWeb/HTML/EventHandler.h

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2021, Andreas Kling <kling@serenityos.org>
+ * Copyright (c) 2021-2022, Andreas Kling <kling@serenityos.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -8,33 +8,29 @@
 
 #include <AK/String.h>
 #include <AK/Variant.h>
-#include <LibJS/Heap/Handle.h>
-#include <LibJS/Runtime/FunctionObject.h>
+#include <LibJS/Heap/Cell.h>
 #include <LibWeb/Bindings/CallbackType.h>
-#include <LibWeb/DOM/DOMEventListener.h>
 
 namespace Web::HTML {
 
-struct EventHandler {
-    EventHandler(String s)
-        : value(move(s))
-    {
-    }
-
-    EventHandler(Bindings::CallbackType c)
-        : value(move(c))
-    {
-    }
+class EventHandler final : public JS::Cell {
+public:
+    explicit EventHandler(String);
+    explicit EventHandler(Bindings::CallbackType&);
 
     // Either uncompiled source code or a callback.
     // https://html.spec.whatwg.org/multipage/webappapis.html#event-handler-value
     // NOTE: This does not contain Empty as part of the optimization of not allocating all event handler attributes up front.
     // FIXME: The string should actually be an "internal raw uncompiled handler" struct. This struct is just the uncompiled source code plus a source location for reporting parse errors.
     //        https://html.spec.whatwg.org/multipage/webappapis.html#internal-raw-uncompiled-handler
-    Variant<String, Bindings::CallbackType> value;
+    Variant<String, Bindings::CallbackType*> value;
 
     // https://html.spec.whatwg.org/multipage/webappapis.html#event-handler-listener
-    RefPtr<DOM::DOMEventListener> listener;
+    DOM::DOMEventListener* listener { nullptr };
+
+private:
+    virtual StringView class_name() const override { return "EventHandler"sv; }
+    virtual void visit_edges(Cell::Visitor&) override;
 };
 
 }