RFR(S) 8220546: Shenandoah Reports timing details for weak root processing

[Zhengyu Gu]

Please review this patch that catches WeakProcessor's timings into 
shenandoah's timings.

Bug: https://bugs.openjdk.java.net/browse/JDK-8220546
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8220546/webrev.00/

Sample output: http://cr.openjdk.java.net/~zgu/JDK-8220546/par_timings.log

Test:
   hotspot_gc_sheanndoah (fastdebug and release)


Thanks,

-Zhengyu