RFR: 8268164: Adopt cast notation for WorkerThread conversions [v3]

[David Holmes]

On Thu, 3 Jun 2021 13:14:54 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

>> Followup of JDK-8267916 (#4240), the same refactoring for `WorkerThread`.
>
> Albert Mingkun Yang has updated the pull request incrementally with one additional commit since the last revision:
> 
>   review

Marked as reviewed by dholmes (Reviewer).

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