RFR: 8296492: Remove ObjectLocker in JVMTI get_subgroups call [v3]

[Coleen Phillimore]

> The JVM code took a ThreadGroup lock before poking into ThreadGroup fields.  Call a method in the ThreadGroup to call the synchronized method instead.
> Tested with tier 1-4.

Coleen Phillimore has updated the pull request incrementally with one additional commit since the last revision:

  fix typo

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Changes:
  - all: https://git.openjdk.org/jdk/pull/11033/files
  - new: https://git.openjdk.org/jdk/pull/11033/files/8615b179..120bee6b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11033&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11033&range=01-02

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/11033.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11033/head:pull/11033

PR: https://git.openjdk.org/jdk/pull/11033