Move heap iteration to object stats to untangle the dependency from
MC.

Bug: v8:7266
Change-Id: I6f0f4f5f3bb0a911591a211ffd71580343765cdd
Reviewed-on: https://chromium-review.googlesource.com/860358Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50502}