RFR: 8065305 - Make it possible to extend the G1CollectorPolicy

[Jesper Wilhelmsson]

Hi,

Please review this small change to allow an extended version of the 
G1CollectorPolicy.
This change also includes a small cleanup to remove a couple of extension points 
in the argument processing.

Bug: https://bugs.openjdk.java.net/browse/JDK-8065305
Webrev: http://cr.openjdk.java.net/~jwilhelm/8065305/

Thanks!
/Jesper