LibWeb: Make MutationObserver GC-allocated

Author: https://github.com/awesomekling
Commit: 905eb8cb4d
Pull-request: https://github.com/SerenityOS/serenity/pull/14816
Reviewed-by: https://github.com/ADKaster
Reviewed-by: https://github.com/linusg ✅

Userland/Libraries/LibWeb/Forward.h

@@ -158,6 +158,7 @@ class ParentNode;
 class Position;
 class ProcessingInstruction;
 class Range;
+class RegisteredObserver;
 class ShadowRoot;
 class StaticNodeList;
 class StaticRange;
@@ -469,7 +470,6 @@ class ImageDataWrapper;
 class IntersectionObserverWrapper;
 class LocationObject;
 class MessageChannelWrapper;
-class MutationObserverWrapper;
 class OptionConstructor;
 class Path2DWrapper;
 class RangePrototype;