RFR: 8264640: CMS ParScanClosure misses a barrier

[Anton Kozlov]

Hi,

Original bug:
   https://bugs.openjdk.java.net/browse/JDK-8264640
   https://github.com/openjdk/jdk13u-dev/commit/efc81a3d

Please review a backport of the fix for CMS. The issue originally manifested in
8u. Original patch applied almost cleanly, except adjustments for very minor
context differences (obj->mark vs obj->mark_raw).

8u webrev:
   http://cr.openjdk.java.net/~akozlov/8264640/8.webrev/

Testing: x86_64 build, hotspot tier1 (2 test failures before and after this patch)

Thanks,
Anton