The recursion between AllocateRaw* and SlowAllocateRaw makes incremental
stepping very complicated. This patch removes the recursion. Follow-on
patches will improve accounting of allocations done by incremental mark.

See: https://codereview.chromium.org/1252053003/#msg5

BUG=
R=hpayer@chromium.org

Review URL: https://codereview.chromium.org/1265443003

Cr-Commit-Position: refs/heads/master@{#29989}