RFR (XS): 8154794,,Add support for experimental fields/events to event-based tracing

[Erik Gahlin]

Hi,

Could I have a review of this small fix that adds support for 
experimental events and experimental fields for event based tracing.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8154794

Webrev:
http://cr.openjdk.java.net/~egahlin/8154794/

Thanks
Erik