RFR: 8298205: Prefer Member Initialization Lists for JFR classes in os_perf.hpp

[Tyler Steele]

Unless there is a reason that I am unaware of for initializing member variables in this way, I believe it is better to use init lists here instead.

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Depends on: https://git.openjdk.org/jdk/pull/11546

Commit messages:
 - Convert to init lists in os_perf.hpp

Changes: https://git.openjdk.org/jdk/pull/11547/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11547&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8298205
  Stats: 22 lines in 1 file changed: 0 ins; 3 del; 19 mod
  Patch: https://git.openjdk.org/jdk/pull/11547.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11547/head:pull/11547

PR: https://git.openjdk.org/jdk/pull/11547