A small step for a JSFunction, one giant leap for V8.

Tbr: clemensb@chromium.org
Bug: v8:8888
Change-Id: I968bb819763994ec611cde7e502adea30339a387
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2315979
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Michael Stanton <mvstanton@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69018}