Clean up tracking new roots, do not track them for CC.

This addresses comments from
    https://android-review.googlesource.com/321552

Test: ART_USE_READ_BARRIER=false testrunner.py -b --host
Test: ART_USE_READ_BARRIER=true testrunner.py -b --host
Bug: 30627598
Change-Id: I9740e599fe8170201a3b5f10113bbeb0dee500fe
2 files changed