[15] 8246612: Shenandoah: add timing tracking to ShenandoahStringDedupRoots

[Zhengyu Gu]

Please review this small patch that adds worker timing for 
ShenandoahStringDedupRoots.


Bug: https://bugs.openjdk.java.net/browse/JDK-8246612
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8246612/weberv.00/index.html

Test:
   hotspot_gc_shenandoah

Thanks,

-Zhengyu