This prevents accumulation of non-regular chunks if unmapper tasks
are not making progress.

Bug: chromium:934453
Change-Id: I552bc4f566f4be8877d9e806cca2aa9c284a7f4f
Reviewed-on: https://chromium-review.googlesource.com/c/1483055Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59842}