This fixes a potential data race that can happen when a freshly
allocated layout descriptor is set on an existing map while the
concurrent marker is using the map to visit an object.

Change-Id: If8ff1c9368b24beb15fefe4a78a21e0f0784d2e8
Reviewed-on: https://chromium-review.googlesource.com/c/1278726Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56604}