[14] RFR 8236902: Shenandoah: Missing string dedup roots in all root scanner

[Roman Kennke]

Hi Zhengyu,

the change looks good! Thanks!

Roman

> String dedup roots are missing in all roots scanner.
> 
> The problem was discovered while running TestHeapDump.java test with
> StringDeduplication enabled, let's add a test there to make sure.
> 
> Bug: https://bugs.openjdk.java.net/browse/JDK-8236902
> Webrev: http://cr.openjdk.java.net/~zgu/JDK-8236902/webrev.00/index.html
> 
> Test:
>   hotspot_gc_shenandoah (fastdebug and release)
> 
> Thanks,
> 
> -Zhengyu
>