RFR: 8199918: Shorten names of CollectedHeap::Name members

[Per Liden]

I propose that we shorten the member names in the CollectedHeap::Name 
enum. This is a pure cosmetic change. Essentially naming the GCs, rather 
than some of the CollectedHeap childen.

From:
SerialHeap
ParallelScavengeHeap
G1CollectedHeap
CMSHeap

To:
Serial
Parallel
CMS
G1

This is a preparatory cleanup for JDK-8199925 and JDK-8199927.

Bug: https://bugs.openjdk.java.net/browse/JDK-8199918
Webrev: http://cr.openjdk.java.net/~pliden/8199918/webrev.0

/Per