This fixes a race between the sweeper and the array buffer tracker
that causes the sweeper to skip unswept pages.

The scenario:
1. Mark-compact GC adds page p to the sweeping_list_ of the sweeper.
2. GC finishes, the main thread starts executinng JS.
3. The main thread takes p->mutex to unregister an array buffer.
4. A sweeper thread removes p from the sweeping_list_ and tries to
   take p->mutex. The try fails. The sweeper drops p and continues
   to the next page.
5. During selection of evacuation candidate in the next GC we hit
   page->SweepingDone() assert.

BUG=chromium:650314

Review-Url: https://codereview.chromium.org/2484153004
Cr-Commit-Position: refs/heads/master@{#40857}