rev 49953 : imported patch 8201491-precleaning
*** 49,58 **** --- 49,59 ---- "CONCURRENT_CYCLE", "CLEAR_CLAIMED_MARKS", "SCAN_ROOT_REGIONS", "CONCURRENT_MARK", "MARK_FROM_ROOTS", + "PRECLEAN", "BEFORE_REMARK", "REMARK", "REBUILD_REMEMBERED_SETS", "CLEANUP_FOR_NEXT_MARK", };