RFR: 8220602: Shenandoah-SA: Enable best-effort implementation of heap walk

[Zhengyu Gu]

Hi Roman,

Thanks for reviewing.

>>    Passed submit test.
> 
> It looks mostly ok. But why do we need the change in CollectedHeap? Why 
> not keep this an impl detail of Shenandoah heap?

Because the region iteration code is in shared (ObjectHeap.java)

Thanks,

-Zhengyu

> 
> Roman
>