During off-thread space merge, we free the linear allocation area in the
off-thread space. Since the off-thread space isn't marked, we have to
make sure that we don't try to compensate for black allocated live bytes.

Bug: chromium:1011762
Change-Id: Id2eb2212dc25e78952f817482abcdb4b49f3a373
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2111224Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66823}