RFR (trivial) 8214356: Verification of class metadata unloading takes a long time

[coleen.phillimore at oracle.com]

Summary: conditionalize verify_chunk_in_freelist call.

Tested with stress test that exercises this code: 
test/hotspot/jtreg/vmTestbase/metaspace/stressDictionary

open webrev at http://cr.openjdk.java.net/~coleenp/8214356.01/webrev
bug link https://bugs.openjdk.java.net/browse/JDK-8214356

Thanks,
Coleen