Replaces the old PromotionQueue that was kept at the end of to space.

This change allows a future implementation to use 
(a) a thread-local promotion list and 
(b) enables work stealing for concurrent scenarios.

Bug: chromium:738865
Change-Id: I4c983a36e69ad4a9462eb9a59f586a85e51acbde
Reviewed-on: https://chromium-review.googlesource.com/561141
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46446}