RFR: 8296492: Remove ObjectLocker in JVMTI get_subgroups call [v5]

[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:

  Forgot a null check.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/11033/files
  - new: https://git.openjdk.org/jdk/pull/11033/files/89955d30..5bfec2e9

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11033&range=04
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11033&range=03-04

  Stats: 2 lines in 1 file changed: 1 ins; 1 del; 0 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