Add an assertion that allocated_bytes >= 0 in IncrementalMark::Step and then
make it pass. We were not being diligent in maintaining top_on_previous_step_
and as a result inaccurate, and even negative values of allocated_bytes were
being reported to Step.

BUG=
R=hpayer@chromium.org

Review URL: https://codereview.chromium.org/1274453002

Cr-Commit-Position: refs/heads/master@{#30778}