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

[Zhengyu Gu]

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