Request for Review (s) - 8157620: Guarantee in run_task(task, num_workers) fails

[Jon Masamitsu]

https://bugs.openjdk.java.net/browse/JDK-8157620

A phase of a G1 collection chooses its own number of GC workers
but doesn't add more workers if more are needed than have been
created.   Add additional workers as needed.  Also changed the
guarantee to check the number of workers against the total number
of workers since the number of GC workers used to execute the
task may be different than active_workers (as in this case).

Added an execution of TestGCOld with parameters that evoked the
failure.

http://cr.openjdk.java.net/~jmasa/8157620/webrev.00/

Tested fix with TestGCOld.
Stability tested with gc_test_suite

Thanks.

Jon