RFR: 8269261: The PlaceHolder code uses Thread everywhere but is always dealing with JavaThreads

[David Holmes]

Please review this trivial cleanup to change Thread to JavaThread.

Testing (in progress):
  - tiers 1-3 sanity check
  - GHA

Thanks,
David

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Commit messages:
 - 8269261: The PlaceHolder code uses Thread everywhere but is always dealing with JavaThreads

Changes: https://git.openjdk.java.net/jdk/pull/4592/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4592&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8269261
  Stats: 20 lines in 2 files changed: 0 ins; 0 del; 20 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4592.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4592/head:pull/4592

PR: https://git.openjdk.java.net/jdk/pull/4592