RFR(xs): 8080584: isGCActiveMark.hpp should not include parallelScavengeHeap.hpp

[Per Liden]

Hi,

This is a follow up patch to the GC directory restructure, which removes 
the unnecessary dependency between gc/shared/isGCActiveMark.hpp and 
gc/parallel/parallelScavengeHeap.hpp

Webrev: http://cr.openjdk.java.net/~pliden/8080584/webrev.0/
Bug: https://bugs.openjdk.java.net/browse/JDK-8080584

cheers,
/Per