Merge OLD_TO_OLD and OLD_TO_NEW per page. This enables removing atomic
operations for the slot updates, effectively removing the need for
fences.

Bug: chromium:651354
Change-Id: I9e318bef06c403b135d638cf94fda9569dcf0e36
Reviewed-on: https://chromium-review.googlesource.com/539338
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46014}