RFR (XS) 8232205: Shenandoah: missing "Update References" -> "Update Roots" tracing

[Roman Kennke]

Ok. Thanks!

Roman


> Bug:
>   https://bugs.openjdk.java.net/browse/JDK-8232205
> 
> Noticed that -Xlog:gc+stats does not print "Update Roots" section for "Update References". This is a
> regression since JDK-8223951.
> 
> Fix:
>   https://cr.openjdk.java.net/~shade/8232205/webrev.01/
> 
> Testing: hotspot_gc_shenandoah, eyeballing gc+stats
>