RFR: 8139892: Allow G1CollectorPolicy to specify if reference processing should be enabled

[Erik Helin]

Hi all,

this patch adds the method `bool should_process_references() const` to
G1CollectorPolicy and also updates G1CollectedHeap to make use of it.
This enables a G1CollectorPolicy to make decisions about whether
references should be processed or not.

Enhancement:
https://bugs.openjdk.java.net/browse/JDK-8139892

Webrev:
http://cr.openjdk.java.net/~ehelin/8139892/webrev.00/

Testing:
JPRT

Thanks,
Erik