JMC-5640: Used Heap After GC Chart

[Ken Dobson]

Hi all,

Here is a patch that adds a Used Heap Post GC chart to the Garbage
Collections page.
The implementation is a bit hacky but the only way I could find to add a
variation of an attribute already being displayed on a page was to create a
duplicate attribute in JdkAttributes.java as well as compare names for
uniqueness as identifiers are no longer unique if we intend to look at
different subsets of the same attribute.

If anyone has any ideas as to how to enact this change in a way that allows
us to maintain consistency with the way the other pages operate as well as
a way to avoid multiple copies of attributes feel free to share them.

Bug Id: https://bugs.openjdk.java.net/browse/JMC-5640
Webrev: http://cr.openjdk.java.net/~kdobson/JMC-5640/webrev/

Thanks,

Ken