This patch hooks up concurrent marking (behind the flag) with the rest
of the GC:
1. Incremental marking spawns concurrent marking task seeded with the
root set.
2. Mark-compact waits for concurrent marking tasks to finish.
3. Scavenger does fast promotion if concurrent marking is pending.

BUG=chromium:694255

Review-Url: https://codereview.chromium.org/2735803005
Cr-Commit-Position: refs/heads/master@{#44526}