This subsumes the old behavior of --allow-natives-for-fuzzing under
--fuzzing as well. Both flags are used in a redundant way in fuzz
configs. Only --allow-natives-for-fuzzing wasn't specified as a
required argument, leading to the bug below.

We still need the flag --allow-natives-for-differential-fuzzing
to allow different functions when using differential fuzzing.

Bug: chromium:1094866
Change-Id: I398791779e58ed4d80e896c1cfea343848159212
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2246568
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68401}