RFR: 8252888: Simplify G1MMUTracker class hierarchy [v2]

[Kim Barrett]

> On Sep 22, 2020, at 4:02 AM, Albert Mingkun Yang <ayang at openjdk.java.net> wrote:
> 
> On Mon, 21 Sep 2020 22:02:16 GMT, John Tortugo <github.com+2249648+JohnTortugo at openjdk.org> wrote:
> 
>>> Relates to: https://bugs.openjdk.java.net/browse/JDK-8252888
>>> Tested on: x86_64 - Linux - tier1
>>> Need sponsor: yes
>>> 
>>> Merge `G1MMUTracker` and `G1MMUTrackerQueue` into a single class and make method members non virtual.
>> 
>> John Tortugo has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>  Rename MMUTrackerQueue[Element] to MMUTracker
> 
> Thank you for the update. If you feel like it, `static consexpr uint QueueLength = 64;` reads better than `enum
> PrivateConstants { ... }`, IMO. Not sure why an `enum` was used in the first place. The rest look fine to me.

Regarding use of enum, it’s historical: https://bugs.openjdk.java.net/browse/JDK-8153116