lib: update class fields TODO in abort_controller.js

https://bugs.chromium.org/p/v8/issues/detail?id=10704 is already
fixed, but since AbortController currently throws ERR_INVALID_THIS
we'll revert to class fields in a subsequent patch. For now just
update the comments.

PR-URL: nodejs#42361
Refs: nodejs@b1c3909
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Mestery <mestery@protonmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>

lib/internal/abort_controller.js

@@ -291,9 +291,8 @@ function abortSignal(signal, reason) {
   signal.dispatchEvent(event);
 }
 
-// TODO(joyeecheung): V8 snapshot does not support instance member
-// initializers for now:
-// https://bugs.chromium.org/p/v8/issues/detail?id=10704
+// TODO(joyeecheung): use private fields and we'll get invalid access
+// validation from V8 instead of throwing ERR_INVALID_THIS ourselves.
 const kSignal = Symbol('signal');
 
 function validateAbortController(obj) {