RFR: 8252476: as_Worker_thread() doesn't check what it intends

[Albert Mingkun Yang]

Convert `as_Worker_thread` to non-virtual, like other `as_*_thread` methods.

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Commit messages:
 - as_worker_thread

Changes: https://git.openjdk.java.net/jdk/pull/4167/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4167&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8252476
  Stats: 11 lines in 2 files changed: 5 ins; 5 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4167.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4167/head:pull/4167

PR: https://git.openjdk.java.net/jdk/pull/4167