The concurrent marker now colors objects grey and black using
the atomic markbit operations.

The heap visitor is changed in two ways:
1) It iterates the map pointer of each object.
2) It guards object visitation with a predicate, which is overridden
in the concurrent marker with the result of GreyToBlack transition.

BUG=chromium:694255

Review-Url: https://codereview.chromium.org/2855003004
Cr-Commit-Position: refs/heads/master@{#45099}