That's possible because JS builtins are JSFunctions that embed a
NativeContext.

Bug: v8:7793
Change-Id: Id2bf7844fcfb53df733100f1e3e554f25a78482a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1926150Reviewed-by: Michael Stanton <mvstanton@chromium.org>
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65068}