RFR: 8271514: support JFR use of new ThreadsList::Iterator

[Daniel D.Daugherty]

A trivial fix to support JFR use of new ThreadsList::Iterator.

This fix was tested with Mach5 Tier[1-3].

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Depends on: https://git.openjdk.java.net/jdk/pull/4948

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

Changes: https://git.openjdk.java.net/jdk/pull/4949/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4949&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8271514
  Stats: 39 lines in 2 files changed: 20 ins; 11 del; 8 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