Enable the computation and testing of GC metrics in ART's CMC collector. am: 68643f562e

Original change: https://android-review.googlesource.com/c/platform/art/+/2451425

Change-Id: I4ae24023dd04c74561513e82869724ed47d2e2ff
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>