This reverts commit 9cbb2ed4.

Reason for revert: Causes flakiness, see http://shortn/_FEVutYBGP7

Original change's description:
> [heap] Removed keep-one-unused-page concept in sweeper.
> 
> This works because we pool regular non-executable pages on a lower level. Executable pages are currently not supported by the pooling mechanism. If this regresses we should fix it.
> 
> Change-Id: Ief3484d59f1f1f4bc63f8e718482e4174bedc012
> Reviewed-on: https://chromium-review.googlesource.com/778939
> Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
> Commit-Queue: Hannes Payer <hpayer@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#49536}

TBR=hpayer@chromium.org,mlippautz@chromium.org

Change-Id: If46fe713f1b1440246803e110838a3958f21dcdf
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/785090Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49565}