If sweeping is not making progress and there are many young generation
GCs happening, then this can lead to accumulation of memory chunks in
the unmapper queue.

Bug: chromium:771966
Change-Id: Ief73ada0d17198a80b668850c6d2e7ea413113e7
Reviewed-on: https://chromium-review.googlesource.com/702479Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48312}