RFR: 8020692: TestGCEventMixed.java failed because of timestamp in event after end event

[Erik Helin]

Hi all,

this small change ensures that an eventual concurrent mode failure trace
event for G1 always is sent before getting a timestamp for the GC trace
event. This change can be seen in g1CollectedHeap.cpp.

I also updated the code in gcTraceSend.cpp to always explicitly set the
endtime with os::elapsed_counter to ensure that the same timer is used
for all GC trace events.

Webrev:
http://cr.openjdk.java.net/~ehelin/8020692/webrev.00/

Bug:
http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=8020692

Testing:
- JPRT
- The GC trace tests

Thanks,
Erik