[sh/jdk8u] Process weak roots for class unloading cycles

[Roman Kennke]

Looks good to me. Thank you!

Roman

> In jdk8u, Shenandoah marks through weak roots if class unloading is off 
> for the cycle, so that, Shenandoah only needs to process weak roots for 
> class unloading cycle.
> 
> Currently, we have following logic:
> 
> 490   if (_heap->process_references()) {
> 491     weak_refs_work(full_gc);
> 492   } else {
> 493     weak_roots_work(full_gc);
> 494   }
> 
> which suggests weak roots have something to do with reference 
> processing, that seems wrong. I thought we have corrected this before, 
> maybe in 11u?
> 
> Webrev: 
> http://cr.openjdk.java.net/~zgu/shenandoah/weak_roots_processing/webrev/
> 
> Test:
>   hotspot_gc_shenandoah
> 
> Thanks,
> 
> -Zhengyu
>