This fixes a bug from 0400fc20,
which assumed that we can set the markbits and the marked
descriptors counter independently. This does not work because
the Scavenger skips slots recording for non-black promoted objects.

The fix is to mark the descriptor array black whenever we change
the marked descriptors counter.

Bug: v8:8617, v8:8618, v8:8486
Tbr: mlippautz@chromium.org
Change-Id: I80f3488061fa648b6c81963ba802ef045d92bcc6
Reviewed-on: https://chromium-review.googlesource.com/c/1387486
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58417}