RFR (XXS): 8233997: Some members of HeapRegion are not cleared in HeapRegion::hr_clear()

[Thomas Schatzl]

Hi all,

   can I get reviews for this small change that fixes some 
reinitialization problem of HeapRegions with the young remset sampling 
thread?

So the young remset sampling thread recalculates young gen remset sizes, 
and it happens that without this fix it may read the (old) remset size 
of the previous use of a given young gen region (also because of 
JDK-8233998).

The fix is to properly clear the remaining members not cleared yet in 
hr_clear.

CR:
https://bugs.openjdk.java.net/browse/JDK-8233997
Webrev:
http://cr.openjdk.java.net/~tschatzl/8233997/webrev/
Testing:
hs-tier1-5 with other similar patches, specjvm2008 specjvm.validation 
with -XX:G1ConcRefinementServiceIntervalMillis=1 (that one causes issues 
quickly if you assert that the remembered sets for regions only grows)

Thanks,
   Thomas