RFR: 8165455: Tracing events for G1 have incorrect metadata

[Erik Helin]

Hi all,

this small patch updates some names, units, labels and description for
three G1 trace events: G1MMU, G1BasicIHOP and G1AdaptiveIHOP.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8165455

Webrev:
http://cr.openjdk.java.net/~ehelin/8165455/00/

Testing:
- JPRT

Thanks,
Erik