Use relaxed reads to allow changing the pointer concurrently in the 
GC. The layout descriptor will either be fast and there's nothing
to be done, or it will be slow, and we will use the forwarding
pointer to determine the proper version to use.

Bug: chromium:726040
Change-Id: I0a376752c3a99abf0874070387fcaeb3cee0dcb2
Reviewed-on: https://chromium-review.googlesource.com/519346
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45652}