Disable heap iteration for Shenandoah in SA

[Zhengyu Gu]

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?

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

Thanks,

-Zhengyu

> 
> Testing: hotspot_gc_shenandoah, hotspot_serviceability
> 
> Ok?
> 
> 
>