RFR (S) 8244326: Shenandoah: global statistics should not accept bogus samples

[Roman Kennke]

> Bug:
>   https://bugs.openjdk.java.net/browse/JDK-8244326
> 
> Fix:
>   https://cr.openjdk.java.net/~shade/8244326/webrev.01/
> 
> Testing: hotspot_gc_shenandoah, eyeballing the logs

Why not declare uninitialized a const field?

Thanks,
Roman