MinorMC only used a single color (grey) while the full MC used 2 colors
(grey and black). Update MinorMC to use black as well. This aligns and
brings full MC and MinorMC closer, and allows to reuse more of the
existing sweeping infrastructure for the non-moving MinorMC.

Bug: v8:12612
Change-Id: Ifa740537c4587dc197196e41829ea74a312b79d0
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3683320Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80894}