8142403: Make G1CollectorPolicy::predictor const

[Erik Helin]

Hi all,

this small patch makes the method G1CollectorPolicy::predictor const.
The method can also return a const reference to the G1Predictor since
the only public method on G1Predictor is const.

Enhancement:
https://bugs.openjdk.java.net/browse/JDK-8142403

Webrev:
http://cr.openjdk.java.net/~ehelin/8142403/webrev.00/

Testing:
JPRT

Thanks,
Erik