RFR(S) 8227927: Move ParallelCleaningTask back to G1

[Thomas Schatzl]

Hi Zhengyu,

On Tue, 2019-07-23 at 15:04 -0400, Zhengyu Gu wrote:
> ParallelCleaningTask was moved from G1 to shared directory, so 
> Shenandoah can use it. Now, Shenandoah no longer uses it, it can
> return to G1.
> 
> However, Shenandoah still uses some of sub-tasks defined in 
> shared/parallelCleaning.hpp/cpp, therefore, only
> ParallelCleaningTask and JVMCICleaningTask are moved to G1.
> 
> Bug: https://bugs.openjdk.java.net/browse/JDK-8227927
> Webrev: 
> http://cr.openjdk.java.net/~zgu/JDK-8227927/webrev.00/index.html
> 
> Test:
>    hotspot_gc (fastdebug and release) on Linux x86_64.
>    Submit tests in progress
> 

  looks good, thanks a lot.

Thomas