RFR: 8280870: Parallel: Simplify CLD roots claim in Full GC cycle

[Albert Mingkun Yang]

Simple change around CLD roots claim logic in `MarkFromRootsTask`.

Test: hotspot_gc

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Commit messages:
 - ps-cld-root

Changes: https://git.openjdk.java.net/jdk/pull/7265/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7265&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8280870
  Stats: 50 lines in 1 file changed: 11 ins; 34 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7265.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7265/head:pull/7265

PR: https://git.openjdk.java.net/jdk/pull/7265