RFR: 8078901: Add trace event for G1 MMU information

[Stefan Johansson]

Hi,

Please review this change to add a trace event for MMU tracking:
https://bugs.openjdk.java.net/browse/JDK-8078901

Webrev:
http://cr.openjdk.java.net/~sjohanss/8078901/hotspot.00/

Summary:
When adding pause information to the G1MMUTracker, it now reports a 
trace-event with the latest MMU information.

add_pause() previously took a bool argument that was unused, but said 
whether or not it was a concurrent-cycle pause. I removed the bool 
argument and added G1MMUTracer* argument. The G1MMUTracer is an 
interface that both the stw-tracer and the cm-tracer now implement to 
make sure the correct GC id is provided with the event.

Thanks,
Stefan