With the exception for map space we can avoid atomic access at all since
pages are locked.

Map space is different since it contains old to new poitners to
LayoutDescriptors that are concurrently used by other tasks for
iterating objects.

Bug: chromium:651354
Change-Id: If7ed99d21676bad8d2944132fb9696ff4123624d
Reviewed-on: https://chromium-review.googlesource.com/539642
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46031}