RFR (S): 8006954: GC Cause equals No GC for CMS background collection in the trace GC event

[Erik Helin]

Hi all,

this change makes sure that the "cause" field in the 
/vm/gc/collection/garbage_collection event is set for a CMS background 
collection. This is done by ensuring that the _cause field in 
GenCollectedHeap is set before registering the start of a GC event.

Webrev:
http://cr.openjdk.java.net/~ehelin/8006954/webrev.00/

Bug:
http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=8006954

Thanks,
Erik