RFR: 8175375: MemoryPoolMXBean.getCollectionUsage() does not works with -XX:+UseG1GC -XX:+ExplicitGCInvokesConcurrent

[Fairoz Matte]

Hi,

Please review this change.
MemoryPoolMXBean.getCollectionUsage() does not works in G1 with "-XX:+ExplicitGCInvokesConcurrent", it always returns 0.
In case of "+ExplicitGCInvokesConcurrent" call to heap usage collection doesn't honor full gc.
With the change of  8175375, it will check for full GC request and set the heap usage collection accordingly.

JBS issue - https://bugs.openjdk.java.net/browse/JDK-8175375 
Webrev - http://cr.openjdk.java.net/~fmatte/8175375/webrev.00/

Testing:
mach5 tier1-3

Thanks,
Fairoz