RFR: 8269261: The PlaceHolder code uses Thread everywhere but is always dealing with JavaThreads [v3]

[David Holmes]

> Please review this trivial cleanup to change Thread to JavaThread.
> 
> Testing (in progress):
>   - tiers 1-3 sanity check
>   - GHA
> 
> Thanks,
> David

David Holmes has updated the pull request incrementally with one additional commit since the last revision:

  Fixed alignment

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/4592/files
  - new: https://git.openjdk.java.net/jdk/pull/4592/files/d84e386d..755222da

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=4592&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=4592&range=01-02

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 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