LayoutDescriptor is currently the only case where the Scavenger needs to
potentially follow an updated slot to iterate an object. This scenario
requires at least Acq/Rel semantics.

In order to use relaxed store/load for the slots we need to allocate it
pretenured.

Bug: chromium:738865
Change-Id: I353fa6f252b436918b60fff54ece5ce5c7783072
Reviewed-on: https://chromium-review.googlesource.com/590429Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46965}