- The barrier for scavenge only checked for whether new space pages were swept.
  This is not enough as a concurrent task could still hang right before trying to
  lock the page for sweeping. Remove the barrier completely.
- Avoid unmapping of new space pages while sweeping using a delayed list that
  gets emptied upon the next call to the unmapper.

BUG=chromium:628984
R=hpayer@chromium.org

Review-Url: https://codereview.chromium.org/2250423002
Cr-Commit-Position: refs/heads/master@{#38710}