RFR(XS): 8189274: Allow cutoff attribute for event based tracing

[Erik Gahlin]

Hi,

Could I have a review of this small change that will add the capability 
to use a cutoff attribute on an event.

Webrev:
http://cr.openjdk.java.net/~egahlin/8189274/

Bug:
https://bugs.openjdk.java.net/browse/JDK-8189274

Thanks
Erik