- Clear flags to avoid the quite expensive query for whether this page
  is to be swept.
- Use a vector instead of a list as we always expect a small number of
  pages to go through the pool and we want to avoid memory management on
  this path.

BUG=

Change-Id: If3c0ad480b8e4f3ccf5a0ef43200c5269822245d
Reviewed-on: https://chromium-review.googlesource.com/443248
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43230}