RFR: 8255457: Shenandoah: cleanup ShenandoahMarkTask [v2]

[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.

Aleksey Shipilev has updated the pull request incrementally with one additional commit since the last revision:

  Fix whitespace

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/883/files
  - new: https://git.openjdk.java.net/jdk/pull/883/files/cb954b7f..12a0927e

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=883&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=883&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 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