Objects that reside below the age mark could be on pages that have been moved
within new space. In this case mementos survived which can actually point to
already-collected allocation sites.

BUG=chromium:631050,chromium:581412
R=hpayer@chromium.org

Review-Url: https://codereview.chromium.org/2179033005
Cr-Commit-Position: refs/heads/master@{#38094}