PCScan: Fix race in scan scheduling heuristic to avoid useless scans.

Instead of comparing the scan limit with the current quarantine size,
we decrement the limit. The first deallocation that makes it negative
triggers the scan. With that the state is atomic.

Bug: 11297512
Change-Id: I50516866913984ad69041efc269471870f4097a7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2774979


Commit-Queue: Hannes Payer <hpayer@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Anton Bikineev <bikineev@chromium.org>
Cr-Commit-Position: refs/heads/master@{#866187}