Expose kSupportsCompaction to be able to refer to it from other traits.

Change-Id: I3a0870853fabfac993eff22886a0a31a52d90055
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2653225
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Auto-Submit: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72372}