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

[Zhengyu Gu]

ShenandoahInitMarkRootsClosure is only used by STW mark, let's move it there.

Test:
- [x] hotspot_gc_shenandoah

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Commit messages:
 - JDK-8265995

Changes: https://git.openjdk.java.net/jdk/pull/3701/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3701&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8265995
  Stats: 49 lines in 4 files changed: 24 ins; 25 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3701.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3701/head:pull/3701

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