This moves scavenging functionality into a separate component so that
neither the scavenger nor objects-visiting need to be exposed outside
the heap.

R=hpayer@chromium.org,mlippautz@chromium.org

Review URL: https://codereview.chromium.org/1323993004

Cr-Commit-Position: refs/heads/master@{#30712}

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}

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}