RFR: JDK-8148734: G1: Make G1GCPhaseTimes keep track of the start GC time

[Bengt Rutisson]

Hi everyone,

Could I have a couple of reviews for this change?

http://cr.openjdk.java.net/~brutisso/8148734/webrev.00/
https://bugs.openjdk.java.net/browse/JDK-8148734

Currently G1CollectedHeap is responsible for keeping track of the start 
time for a GC and pass it along to G1GCPhaseTimes when it is time to 
print the G1 log details.

Since G1GCPhaseTimes has both a note_gc_start() and a note_gc_end() it 
seems more stable that G1GCPhaseTimes keeps track of the GC time itself.

Thanks,
Bengt