{CommitPageSize()} can be smaller than {AllocatePageSize()} (on win64,
it's 4kb vs 64kb), thus use the commit size where appropriate.

R=titzer@chromium.org

Change-Id: Ic9a009158d788aa0c53e15790ea089f01ade0d0a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1605940Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61450}