RFR: 8280870: Parallel: Simplify CLD roots claim in Full GC cycle [v2]

[Albert Mingkun Yang]

> Simple change around CLD roots claim logic in `MarkFromRootsTask`.
> 
> Test: hotspot_gc

Albert Mingkun Yang has updated the pull request incrementally with one additional commit since the last revision:

  review

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/7265/files
  - new: https://git.openjdk.java.net/jdk/pull/7265/files/67fe7ff2..285b537a

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=7265&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=7265&range=00-01

  Stats: 10 lines in 1 file changed: 0 ins; 5 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