RFR: 8268164: Adopt cast notation for WorkerThread conversions

[Albert Mingkun Yang]

Followup of JDK-8267916 (#4240), the same refactoring for `WorkerThread`.

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Commit messages:
 - worker_thread

Changes: https://git.openjdk.java.net/jdk/pull/4334/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4334&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8268164
  Stats: 14 lines in 4 files changed: 5 ins; 8 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4334.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4334/head:pull/4334

PR: https://git.openjdk.java.net/jdk/pull/4334