[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

Committed: https://crrev.com/310f2ece0f78d698e606e8166cfb13107ebc65d2
Cr-Commit-Position: refs/heads/master@{#30548}

[Michael Lippautz, 2015-09-02 16:55:44]

lgtm, thanks a lot!

[Michael Starzinger, 2015-09-02 16:57:40]

The CQ bit was checked by mstarzinger@chromium.org

[commit-bot: I haz the power, 2015-09-02 16:57:48]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1310953008/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1310953008/1

[commit-bot: I haz the power, 2015-09-02 17:09:49]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-09-02 17:10:07]

Patchset 1 (id:??) landed as
https://crrev.com/310f2ece0f78d698e606e8166cfb13107ebc65d2
Cr-Commit-Position: refs/heads/master@{#30548}