The deadlock can happen when two scavenging tasks process two different
pages for their old->new sets and at the same time try to allocate in
old space which triggers sweeping of the other task's page.

Bug: v8:6754
Change-Id: I6087553631e198d5ecfb8ab37925ac41cd6995bd
Reviewed-on: https://chromium-review.googlesource.com/635843
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47610}