When switching from atomicops to std::atomic, I introduced a bug that
makes us miss the actual peak memory consumption, and only report the
second-highest memory usage in each zone.
This CL fixes that.

R=petermarshall@chromium.org

Bug: chromium:939724
Change-Id: I3db8eeb9719eb026cf6b4a9690c702cbc4c11bd9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1541227Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60505}