RFR: 8254562: ZGC: Remove ZMarkRootsTask [v3]

[Per Liden]

On Mon, 12 Oct 2020 14:04:20 GMT, Stefan Karlsson <stefank at openjdk.org> wrote:

>> After introducing concurrent stack scanning, we don't need to mark through any roots during the mark start pause.
>> Remove the code.
>> Note that the old code passed in `false` to `_roots(false /* visit_jvmti_weak_export */)`. This has the effect that no
>> roots are visited by the iterator: void ZRootsIterator::oops_do(ZRootsIteratorClosure* cl) {
>>   ZStatTimer timer(ZSubPhasePauseRoots);
>>   if (_visit_jvmti_weak_export) {
>>     _jvmti_weak_export.oops_do(cl);
>>   }
>> }
>
> Stefan Karlsson has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Review 1

Marked as reviewed by pliden (Reviewer).

-------------

PR: https://git.openjdk.java.net/jdk/pull/601