This will allow us to concurrently allocate buckets during iteration.

Bug: chromium:738865
Change-Id: I88bd1ac152d1ef8b40395f0ba3e55a7c3e82f75d
Reviewed-on: https://chromium-review.googlesource.com/575990
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46737}