[sh/jdk8u] RFR Shenandoah: missing SystemDictionary roots in ShenandoahHeapIterationRootScanner

[Zhengyu Gu]

Please review this patch that adds SystemDictionaryRoots to 
ShenandoahHeapIterationRootScanner. It appears to be the cause of recent 
TCK 8 test failures.

Webrev: 
http://cr.openjdk.java.net/~zgu/shenandoah/heap_itr_scanner/webrev.00/index.html

Test:
   hotspot_gc_shenandoah

   Imported Xin Liu's vmTestbase 8u backport, ran vmTestbase/nsk/jdi
   test. Reproducible before the patch and passed afterward.

Thanks,

-Zhengyu