RFR: 8035667: EventMetaspaceSummary doesn't report committed Metaspace memory

[Erik Helin]

Hi all,

this small patch makes the the trace event vm/gc/heap/metaspace_summary 
report the amount of committed memory instead of capacity. In the 
metaspace code, committed is used in all places except one (and that 
will soon change) when making decisions about sizing etc, therefore it 
makes much more sense that the trace event reports the amount of 
committed memory.

Webrev:
http://cr.openjdk.java.net/~ehelin/8035667/webrev.00/

Testing:
- Running JTREG JFR tests.

Thanks,
Erik