During incremental marking we need to clear to old generation
mark bits for promoted pages.

Bug: chromium:651354
Change-Id: I986de08d6a123b2eff3dd7824729cc1cfe29aeba
Reviewed-on: https://chromium-review.googlesource.com/522665Reviewed-by: Hannes Payer <hpayer@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45683}