RFR: 8261509: Move per-thread StackWatermark from Thread to JavaThread class

[Zhengyu Gu]

Per-thread StackWatermark only applies to JavaThread, so it should belong to there.

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Commit messages:
 - 8261509: Move per-thread StackWatermark from Thread to JavaThread class

Changes: https://git.openjdk.java.net/jdk/pull/2513/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2513&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261509
  Stats: 11 lines in 1 file changed: 5 ins; 6 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2513.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2513/head:pull/2513

PR: https://git.openjdk.java.net/jdk/pull/2513