[15] RFR 8246593: Shenandoah: string dedup roots should be processed during concurrent weak roots phase

[Zhengyu Gu]

String dedup roots are weak roots, they are mistakenly placed as strong 
roots during weak/strong roots split (JDK-8242643).

This patch moves string dedup roots from concurrent strong roots to 
concurrent weak roots.

Bug: https://bugs.openjdk.java.net/browse/JDK-8246593
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8246593/webrev.00/

Test:
   hotspot_gc_shenandoah with -XX:+UseStringDeduplication

Thanks,

-Zhengyu