This fixes benign data races reported by TSAN when a shared layout
descriptor is updated at the end.

Change-Id: I76662cb5fc2b8ab1728e3d1bf42a55a107442eed
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1709422Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62863}