RFR (8u40) 8044473 : Allow for extended set of platform MXBeans

[Brent Christian]

Please review my change for:

   8044473 - Allow for extended set of platform MXBeans

which adds an internal ExtendedPlatformComponent mechanism.

There are no new public APIs or MXBeans.

JBS Issue:
https://bugs.openjdk.java.net/browse/JDK-8044473

Webrev:
http://cr.openjdk.java.net/~bchristi/8044473/webrev.00/

Thanks,
-Brent