Do a step before selecting the limit for the next step. However, as seen
on crbug.com/795323, while this fix makes us more precise in our
accounting, we do ending up seeing steps more frequently. This ends up
invoking the idle scavenger more frequently. To compensate, we adjust
the idle scavenger step size.

Bug: 
Change-Id: I7bc2b1785a564dee27aa3ce6a5a196efe9eb6283
Reviewed-on: https://chromium-review.googlesource.com/838440
Commit-Queue: Ali Ijaz Sheikh <ofrobots@google.com>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50816}