RFR: 8298205: Prefer Member Initialization Lists for JFR classes in os_perf.hpp [v4]

[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.

Tyler Steele has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:

 - Change copyright year
 - Changes NULL -> nullptr
 - Convert to init lists in os_perf.hpp

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/11547/files
  - new: https://git.openjdk.org/jdk/pull/11547/files/82cbcae8..71125a88

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11547&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11547&range=02-03

  Stats: 9044 lines in 271 files changed: 6068 ins; 2119 del; 857 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