FRF: Wiring GC events to JFR

[Zhengyu Gu]

Shenandoah has yet to wire proper events to JFR, this patch only address 
GC events.

Webrev:
http://cr.openjdk.java.net/~zgu/shenandoah/jfr-event-1/webrev.00/

Test:
   Manual tests. Eyeballed JMC GC related tabs.

Thanks,

-Zhengyu