The method PSScavenge::invoke() can end up doing a young gc, full gc, both, or none.  This is somewhat confusing, especially when calling code has the need to know what was done (7146343 made a small change in this direction).  The policy in this method is better suited to a higher-level class such as ParallelScavengeHeap.

More specifically, the PSScavenge::invoke() method should do a young gc if possible, and return whether or not the young gc was done (nothing more).  A new controlling method in ParallelScavengeHeap should then determine whether to do a full gc, and return to callers an indication of the type of gc(s) done (i.e., an enum).