Object statistics were collected during the mark compact phase. If
an incremental marking happened before mark compact phase then most
of the objects are already visited and hence this phase does not collect
accurate statistics. This cl updates incremental marking pass to collect
object statistics along with mark compact phase.

BUG=

Review-Url: https://codereview.chromium.org/1943423002
Cr-Commit-Position: refs/heads/master@{#36678}