Separates restoring iterability from sweeping a page. Since the set
of pages where iterability needs to be restored is small, it is
possible to wait and block for the task to finish if necessary.

A follow up change can now remove the fragile logic for delaying
unmapping of pages since it is guaranteed that no background task
keeps a reference to a page for restoring iterability.

Bug: chromium:791043
Change-Id: Ifba45594cc586df3c99e1bbb20a13b44c18dd9a1
Reviewed-on: https://chromium-review.googlesource.com/796419
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49826}