RFR: 8140509: Add note_gc_start to G1CollectorPolicy

[Erik Helin]

Hi all,

this tiny patch adds a new utility method to G1CollectorPolicy,
note_gc_start. This patch further hide the G1GCPhaseTimes a bit more
from G1CollectorPolicy.

Enhancement:
https://bugs.openjdk.java.net/browse/JDK-8140509

Webrev:
http://cr.openjdk.java.net/~ehelin/8140509/webrev.00/

Testing:
JPRT

Thanks,
Erik