RFR: 8140508: Add utility method for logging phases to G1CollectorPolicy

[Erik Helin]

Hi all,

this small patch just adds a small utility method to G1CollectorPolicy,
log_phases.

Enhancement:
https://bugs.openjdk.java.net/browse/JDK-8140508

Webrev:
http://cr.openjdk.java.net/~ehelin/8140508/webrev.00/

Testing:
JPRT

Thanks,
Erik