RFR 8225550: Shenandoah: Prevent SH::object_iterate() call's side-effects

[Aleksey Shipilev]

On 6/11/19 6:22 PM, Zhengyu Gu wrote:
> This is another prerequisite for concurrent root processing. It prevents heap iteration from
> touching objects, that are considered unreachable by current GC cycle. Otherwise, LRB may evacuate
> the objects, make them alive from GC point of view.
> 
> The solution is to use the same roots to seed the iteration as ongoing GC cycle.
> 
> Bug: https://bugs.openjdk.java.net/browse/JDK-8225550
> Webrev: http://cr.openjdk.java.net/~zgu/JDK-8225550/webrev.00/

Looks good.

-Aleksey