RFR: 8140489: Introduce shorthand for average_time_ms in G1CollectorPolicy

[Erik Helin]

Hi all,

this small patch introduces the little helper method
G1CollectorPolicy::average_tims_ms instead of calling
g1_phases()->average_time_ms, making the code a bit shorter and easier
to read.

Enhancement:
https://bugs.openjdk.java.net/browse/JDK-8140489

Webrev:
http://cr.openjdk.java.net/~ehelin/8140489/webrev.00/

Testing:
JPRT

Thanks,
Erik