[12u] 8220546: Shenandoah Reports timing details for weak root processing

[Zhengyu Gu]

Please review this 12u backport.

The patch did not apply cleanly, as string table is not processed by 
WeakProcessor in 12u.

Bug: https://bugs.openjdk.java.net/browse/JDK-8220546
Original patch: http://hg.openjdk.java.net/jdk/jdk/rev/8b61a38be0c5
Original review: 
https://mail.openjdk.java.net/pipermail/hotspot-gc-dev/2019-March/025070.html

JDK12u webrev: http://cr.openjdk.java.net/~zgu/JDK-8220546_12u/webrev.00/

Test:
   hotspot_gc_shenandoah on Linux 64.

Thanks,

-Zhengyu