RFR: 8252476: as_Worker_thread() doesn't check what it intends [v2]

[Albert Mingkun Yang]

On Tue, 25 May 2021 06:43:46 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

>> Convert `as_Worker_thread` to non-virtual, like other `as_*_thread` methods.
>
> Albert Mingkun Yang has updated the pull request incrementally with one additional commit since the last revision:
> 
>   review

Moved the definition to the inline file so that `static_cast` can be used.

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