RFR: 8204538: Split ScanClosure and ScanClosureWithParBarrier

[Kim Barrett]

> On Jun 7, 2018, at 6:16 AM, Stefan Karlsson <stefan.karlsson at oracle.com> wrote:
> 
> Hi all,
> 
> Please review this patch to change ScanClosureWithParBarrier to not inherit from ScanClosure.
> 
> http://cr.openjdk.java.net/~stefank/8204538/webrev.01/
> https://bugs.openjdk.java.net/browse/JDK-8204538
> 
> ScanClosureWithParBarrier is the only ExtendedOopClosure sub class that overrides the do_oop functions of a super class. This patch cleans this up to pave the way for:
> 
> https://bugs.openjdk.java.net/browse/JDK-8204540 - Automatic oop closure devirtualization
> 
> thanks,
> StefanK

I think the data members of ScanClosure can now be made private rather than protected.

Otherwise, looks good.  I don’t need a new webrev for that access change.