If sweeping is in progress then we need to filter out slots in free space after
array trimming, because the sweeper will add the free space into free list.

This CL also fixes a bug in SlotSet::RemoveRange.

BUG=chromium:587004
LOG=NO
TBR=hpayer@chromium.org

Review URL: https://codereview.chromium.org/1701963003

Cr-Commit-Position: refs/heads/master@{#34071}