RFR: 8271514: support JFR use of new ThreadsList::Iterator [v2]

[Daniel D.Daugherty]

> A trivial fix to support JFR use of new ThreadsList::Iterator.
> 
> This fix was tested with Mach5 Tier[1-3].

Daniel D. Daugherty 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:

 - Merge branch 'pull/4948' into JDK-8271514
 - Merge branch 'pull/4948' into JDK-8271514
 - 8271514: support JFR use of new ThreadsList::Iterator

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/4949/files
  - new: https://git.openjdk.java.net/jdk/pull/4949/files/056badc2..3c64ea23

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=4949&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=4949&range=00-01

  Stats: 31265 lines in 73 files changed: 29925 ins; 630 del; 710 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4949.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4949/head:pull/4949

PR: https://git.openjdk.java.net/jdk/pull/4949