RFR (XS) - 8159666 Better CDS support for Event-based tracing

[Ioi Lam]

Hi,

Please review this small fix. It allows event-based tracing code to 
properly remove and restore the trace ID during CDS dump time and JVM 
run time.

https://bugs.openjdk.java.net/browse/JDK-8159666
http://cr.openjdk.java.net/~iklam/jdk9/8159666-cds-event-based-tracing/

Thanks
- Ioi