RFR: 8204538: Split ScanClosure and ScanClosureWithParBarrier

[Stefan Karlsson]

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