RFR: 8278551: Shenandoah: Adopt WorkerThread::worker_id() to replace Shenandoah specific implementation [v2]

[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

Zhengyu Gu has updated the pull request incrementally with one additional commit since the last revision:

  Remove unused constant

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/6853/files
  - new: https://git.openjdk.java.net/jdk/pull/6853/files/7f08ec19..92888fe5

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=6853&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=6853&range=00-01

  Stats: 3 lines in 1 file changed: 0 ins; 3 del; 0 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