RFR: JDK-8214756: SA should ignore archived java heap objects that are not in use

[Jini George]

Requesting reviews for a very small change to fix:

https://bugs.openjdk.java.net/browse/JDK-8214756
Webrev: http://cr.openjdk.java.net/~jgeorge/8214756/webrev.00/index.html

The proposed change is to ignore objects whose associated classes are 
unloaded while walking the heap to create a heapdump in SA.

Thanks,
Jini.