Perform a small marking step whenever we allocate a new lab.

Bug: chromium:1056170
Change-Id: I2a09480a254ddb1afc95bf5ef9a70ec27f512d79
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2366698
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69507}