[sh/jdk8u] RFR Shenandoah: add JFR roots to root processor after JFR integration

[Zhengyu Gu]

There was no JFR when Shenandoah root processing was backported, 
therefore, JFR root is missing.


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

Test:
   hotspot_gc_shenandoah (+/- JFR)

Thanks,

-Zhengyu