[heap] Move ObjectStatsVisitor into the proper component.

[heap] Move ObjectStatsVisitor into the proper component.

This is a follow-up to a previous change and moved object statistics
tracking into its own component. It is no longer intertwinded with the
normal marking logic, but separated out into ObjectStatsVisitor.

R=mlippautz@chromium.org

Review URL: https://codereview.chromium.org/1310953008

Cr-Commit-Position: refs/heads/master@{#30548}