After dfc6b4dd the space size can decrease if the sweeper discovers
new fillers added after marking (e.g. by array trimming).

Bug: chromium:756832
Change-Id: Ibf420593bd12a4fe13a1e47f862302025b52ad58
Reviewed-on: https://chromium-review.googlesource.com/620734Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47431}