RFR: 8255457: Shenandoah: cleanup ShenandoahMarkTask

[Aleksey Shipilev]

The code in ObjArrayChunkedTask is rough around the edges, and can be cleaned up. It would be further extended in JDK-8254315, so it makes sense to make the change before the bulk of JDK-8254315 arrives.

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Commit messages:
 - 8255457: Shenandoah: cleanup ShenandoahMarkTask

Changes: https://git.openjdk.java.net/jdk/pull/883/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=883&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8255457
  Stats: 94 lines in 3 files changed: 42 ins; 24 del; 28 mod
  Patch: https://git.openjdk.java.net/jdk/pull/883.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/883/head:pull/883

PR: https://git.openjdk.java.net/jdk/pull/883