RFR: 8278551: Shenandoah: Adopt WorkerThread::worker_id() to replace Shenandoah specific implementation

[Zhengyu Gu]

Shenandoah stores worker_id in ThreadLocalData for the same purpose as JDK-8278351.

After JDK-8278351, Shenandoah should adopt WorkerThread::worker_id() from JDK-8278351 to replace its own implementation.

Test:
- [x] hotspot_gc_shenandoah

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Commit messages:
 - 8278551: Shenandoah: Adopt WorkerThread::worker_id() to replace Shenandoah specific implementation

Changes: https://git.openjdk.java.net/jdk/pull/6853/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6853&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8278551
  Stats: 35 lines in 4 files changed: 2 ins; 27 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6853.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6853/head:pull/6853

PR: https://git.openjdk.java.net/jdk/pull/6853