RFR: 8296492: Remove ObjectLocker in JVMTI get_subgroups call

[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.

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Commit messages:
 - 8296492: Remove ObjectLocker in JVMTI get_subgroups call

Changes: https://git.openjdk.org/jdk/pull/11033/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11033&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8296492
  Stats: 109 lines in 5 files changed: 30 ins; 67 del; 12 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