Disable heap iteration for Shenandoah in SA

[Roman Kennke]

Hi Zhengyu

> On 11/28/18 12:38 PM, Roman Kennke wrote:
>> As Per pointed out in reviews, we cannot safely iterate objects in SA,
>> because current implementation is size-based. This might just crash.
>> Let's do the same as ZGC for now and disable it, and revert the other
>> pieces.
>>
>> http://cr.openjdk.java.net/~rkennke/disable-sa-heapdump/webrev.00/
> 
> Why also removed cell_header_size stuff?

It's not needed anymore, I want to minimize upstream diff and I think
that the object iteration would need to be implemented differently, not
using cell_header_size.

> If you want to minimize diff, can also remove it from CollectedHeap and
> ShenandoahHeap, this is the sole user.

Good, thanks for pointing out:

http://cr.openjdk.java.net/~rkennke/disable-sa-heapdump/webrev.01/

Good now?

Roman