RFR: 8226897: Provide object age with JFR OldObjectSample event

[Erik Gahlin]

Hi,

Could I have a review of this fix that adds object age to the Old Object 
Sample event.

Example usage:

$ jfr print --events OldObjectSample recording.jfr
jdk.OldObjectSample {
   startTime = 19:00:04.543
   allocationTime = 18:59:41.395
   objectAge = 23,148 s
   lastKnownHeapUsage = 983,1 kB
   object =  [
     java.lang.Class Class Name: java.awt.Conditional
   ]
   arrayElements = N/A
   root = N/A
   eventThread = "AWT-EventQueue-0" (javaThreadId = 34)
}
...

Webrev:  http://cr.openjdk.java.net/~egahlin/8226897/
Bug: https://bugs.openjdk.java.net/browse/JDK-8226897
Testing: test/jdk/jdk/jfr

Thanks
Erik