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

[Roman Kennke]

Hi Zhengyu,

>>>    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)


Ah ok.

I have a patch up my sleeve that eliminates the extra word ;-) But I 
suppose you should not wait for it (probably some weeks).

Reviewed!

Roman