8058209: Race in G1 card scanning could allow scanning of memory covered by PLABs Summary: Read _top before _gc_time_stamp in saved_mark_word() with LoadLoad order to ensure we get a consistent view Reviewed-by: brutisso, dcubed, dholmes, stefank

8064811: Use THREAD instead of CHECK_NULL in return statements Reviewed-by: coleenp, simonis, dholmes