Sweeping a page while currently scavenging it is broken as the scavenger
might override the slot it is currently processing.

Bug: chromium:779503
Change-Id: I224a144b84e97a956bf10ba018132c2713e8f78d
Reviewed-on: https://chromium-review.googlesource.com/752081
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49092}