RFR(S) 8154467: Cleanup initialization of GCPolicyCounters

[Mikael Gerdin]

Hi all,

Please review this small cleanup to how GCPolicyCounters are initialized 
and managed.

GCPolicyCounters* counters() is part of the CollectorPolicy API but in 
fact it is not necessary to expose it at such a general level.
G1 has only used the object internally in the policy code and managing 
the policy counters object can be safely moved inside the G1Policy.

ParalellGC has ignored initializing the field in the CollectorPolicy at
all, since it does not really use the CollectorPolicy API.

The Gen collectors can be adapted to access the counters() accessor 
through the GenCollectorPolicy class instead.

Testing: RBT GC Testing
Webrev: http://cr.openjdk.java.net/~mgerdin/8154467/webrev.0/
Bug: https://bugs.openjdk.java.net/browse/JDK-8154467

/Mikael