RFR: 8265995: Shenandoah: Move ShenandoahInitMarkRootsClosure close to its use

[Aleksey Shipilev]

On Mon, 26 Apr 2021 17:55:41 GMT, Zhengyu Gu <zgu at openjdk.org> wrote:

> ShenandoahInitMarkRootsClosure is only used by STW mark, let's move it there.
> 
> Test:
> - [x] hotspot_gc_shenandoah

Looks fine.

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Marked as reviewed by shade (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/3701