RFR: 8080840: Clean up active_workers() asserts

[Stefan Karlsson]

Hi all,

Please, review this patch to clean up code checking and asserting that 
the FlexibleWorkGang and active_workers() have been setup correctly.

http://cr.openjdk.java.net/~stefank/8080840
https://bugs.openjdk.java.net/browse/JDK-8080840

This patch depends on the patch in:
http://mail.openjdk.java.net/pipermail/hotspot-gc-dev/2015-May/013511.html

Thanks,
StefanK