RFR (S) JDK-8149023: Event based tracing should cover safepoint begin and end

[David Simms]

Greetings all,

I have small request for review, for adding event based tracing to 
"SafepointSynchronize::begin()" and "end()"

JBS: https://bugs.openjdk.java.net/browse/JDK-8149023
Webrev: http://cr.openjdk.java.net/~dsimms/8149023/webrev0/

Testing: built all platforms (RBT), local jtreg

Cheers
/David Simms