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
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_chromium_rel_ng

Review-Url: https://codereview.chromium.org/2866473003
Cr-Commit-Position: refs/heads/master@{#45144}