RFR: 8142494: Add extension point to G1EvacuationRootClosures

[Erik Helin]

Hi all,

this patch adds an extension point to the factory function for
G1EvacuationRootClosures.

Enhancement:
https://bugs.openjdk.java.net/browse/JDK-8142494

Webrev:
http://cr.openjdk.java.net/~ehelin/8142494/webrev.00/

Testing:
JPRT

Thanks,
Erik