webrev: thread id in G1GCParPhaseTimesTracker

[Eric Caspole]

Hi everybody,
Yesterday I was chatting with Bengt about trying to encapsulate the GC 
worker thread id so it does not have to be explicitly passed in in so 
many GC methods just to record the timing stats. I made a first cut at 
this here:

http://cr.openjdk.java.net/~ecaspole/phase_time_thread/01/webrev/

More could be done here but I wanted to see if everyone thought it was 
worth continuing with it.
Thanks,
Eric