In the spirit of the full MC, we evacuate and update pointers in parallel for
the young generation.

The collectors are connected during incremental marking when mark bits are
transferred from the young generation bitmap to the old generation bitmap.

The evacuation phase cannot (yet) move pages and relies completely on copying
objects.

BUG=chromium:651354

Review-Url: https://codereview.chromium.org/2796233003
Cr-Commit-Position: refs/heads/master@{#45074}