Currently, probabilities for extra flags are calculated in the correctness
fuzzer harness, which makes the RNG fragile when bisecting backwards, when
the script's config changes during bisection.

This adds the possibility to pass extra flags on command line to the
script. After a grace period, we will migrate the flag calculation to
clusterfuzz.

NOTRY=true

Bug: chromium:813833
Change-Id: I515181847474515089b847f8aaffc7c6560d9390
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1675945Reviewed-by: Tamer Tas <tmrts@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62359}