RFR: 8144145: G1GCPhaseTimes should allow externally accounted time

[Erik Helin]

Hi all,

this small patch allows code to report accounted time to
G1GCPhaseTimes, for example if something takes time that isn't tracked
by G1GCPhaseTimes (code in the runtime, shared code, etc. that
might be logged some other way).

Enhancement:
https://bugs.openjdk.java.net/browse/JDK-8144145

Webrev:
http://cr.openjdk.java.net/~ehelin/8144145/webrev.00/

Testing:
- GC Basher with full verification.

Thanks,
Erik