RFR (S) 8178336: Unnecessary SystemDictionary walk for Protection domain liveness

[coleen.phillimore at oracle.com]

Summary: remove system dictionary walk and pass strong closure for 
!ClassUnloading

See bug for more details:

open webrev at http://cr.openjdk.java.net/~coleenp/8178336.01/webrev
bug link https://bugs.openjdk.java.net/browse/JDK-8178336

Tested with nightly tier2-5 tests and jprt (runs all GCs) and runThese 
with -XX:-ClassUnloading.

Thanks,
Coleen