RFR: 8298521: Rename members in G1MonitoringSupport

[Yude Lin]

Hi,

Can I have review on this trivial change that renames a few members in G1MonitoringSupport for consistency?

Thanks!

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Commit messages:
 - 8298521: Rename members in G1MonitoringSupport

Changes: https://git.openjdk.org/jdk/pull/11623/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11623&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8298521
  Stats: 14 lines in 2 files changed: 0 ins; 0 del; 14 mod
  Patch: https://git.openjdk.org/jdk/pull/11623.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11623/head:pull/11623

PR: https://git.openjdk.org/jdk/pull/11623