This patch makes the SlotSet bucket type non-atomic by default
and explicitly converts buckets to Atomic32/AtomicWord for each
operation.

Change-Id: Ifaa60a53eb68ca579185be23e379995aeeabe343
Reviewed-on: https://chromium-review.googlesource.com/535481
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45971}