RFR: 8204287: Phase timings not updated correctly after JDK-6672778

[Stefan Johansson]

Hi,

Please review this fix to do correct timings in G1.

Issue: https://bugs.openjdk.java.net/browse/JDK-8204287
Webrev: http://cr.openjdk.java.net/~sjohanss/8204287/00/

Summary:
When the partial trimming was introduced a while back the timing code in 
G1 became a bit more complicated. In one of the timer helpers a member 
structure is used before it is destructed and updated. This leads to the 
times being accounted to the wrong phases and there for the G1 
ergonomics doing some less than optimal decisions.

Testing:
* Local testing with jtreg
* Mach5 testing tier 1-3 looks good
* Sanity performance testing is running

Thanks,
Stefan