[8u-dev] RFA: JDK-8215934: G1 Old Gen MemoryPool CollectionUsage.used values don't reflect mixed GC results

[Hohensee, Paul]

Please approve a G1 serviceability bug backport.

JBS Bug: https://bugs.openjdk.java.net/browse/JDK-8215934
CSR: https://bugs.openjdk.java.net/browse/JDK-8215935
Webrev: http://cr.openjdk.java.net/~phh/8215934/webrev.00/

First backport review thread: http://mail.openjdk.java.net/pipermail/hotspot-gc-dev/2018-October/023507.html
Second backport review thread: https://mail.openjdk.java.net/pipermail/hotspot-gc-dev/2019-January/024462.html
Backport reviewers: tschatzl, jcbeyler
Original JBS Bug: https://bugs.openjdk.java.net/browse/JDK-8195115
Original CSR: https://bugs.openjdk.java.net/browse/JDK-8196719
Original review thread: http://mail.openjdk.java.net/pipermail/hotspot-gc-dev/2018-June/022305.html