Request for review 8004883: NPG: clean up anonymous class fix

[Coleen Phillimore]

Summary: Add klass_holder() to return either mirror or class_loader 
depending on if the class is anonymous or not.

Add code cleanup requested by John Rose and StefanK.

open webrev at http://cr.openjdk.java.net/~coleenp/8004883/
bug link at http://bugs.sun.com/view_bug.do?bug_id=8004883   (just 
filed, might not show up yet).

Ran nashorn tests again.

Thanks,
Coleen