The deadlock can happen when we lock a page for processing old to new
references as part of the scavenger while at the same time trying to
lazily sweep another page for retrieving memory. If two tasks decide to
sweep each others pages they will deadlock.

Bug: v8:6754
Change-Id: Ic9fae0eafa5b5a5cb5eaa1c0aac61de24d1b9486
Reviewed-on: https://chromium-review.googlesource.com/636371
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47647}