[heap] Separate ObjectStats out into its own class.

[heap] Separate ObjectStats out into its own class.

Note that this is only pulling out the bookkeeping side of things, the
marking visitor that actually records the statistics should also move
into the ObjectStats class. That will be done as a follow-up.

R=mlippautz@chromium.org

Review URL: https://codereview.chromium.org/1326793002

Cr-Commit-Position: refs/heads/master@{#30547}