RFR: 8234446: Post-CMS workgroup hierarchy cleanup [v3]

[Albert Mingkun Yang]

On Thu, 29 Apr 2021 09:07:18 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

>> Collapse workgroup hierarchy and some general cleanup.
>> 
>> Tested; hotspot_gc
>
> Albert Mingkun Yang has updated the pull request incrementally with one additional commit since the last revision:
> 
>   undo indentation change

There is some preexisting inconsistency regarding the indentation of fields initializers in constructors, e.g. `SubTasksDone` uses 2-space, while `WorkGang` uses 4-space. In order not to make the review process unnecessarily harder, I have kept the original indentation, despite the inconsistency.

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PR: https://git.openjdk.java.net/jdk/pull/3663