Request for review (s) : 8017462: G1: guarantee fails with UseDynamicNumberOfGCThreads

[Jon Masamitsu]

8017462: G1: guarantee fails with UseDynamicNumberOfGCThreads

https://bugs.openjdk.java.net/browse/JDK-8017462

When fewer than the maximum number of threads was being used for
a parallel section, phase times for the threads that did not execute and
averages for the phase were misleading.  The fix passes the active number of
GC threads  to the G1 phase timers.

http://cr.openjdk.java.net/~jmasa/8017462/webrev.00/

Tested with gc_test_suite.