We need to write the slot with a relaxed write. No additional barrier is
needed because the new object has been published using Release_CAS and
reading either value is fine (as long as all initializing stores have
been published).

NOTRY=true

Bug: chromium:738865
Change-Id: Ieeacccf00e0a7e8eb950eee01060bd1963e30a42
Reviewed-on: https://chromium-review.googlesource.com/589007
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46924}