LibWeb: Make TreeWalker GC-allocated

Author: https://github.com/awesomekling
Commit: a4ddb0ef87
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

@@ -604,7 +604,6 @@ class TextDecoderWrapper;
 class TextEncoderWrapper;
 class TextMetricsWrapper;
 class TextWrapper;
-class TreeWalkerWrapper;
 class UIEventWrapper;
 class URLConstructor;
 class URLPrototype;