8296492: Remove ObjectLocker in JVMTI get_subgroups call

Reviewed-by: dholmes, alanb, sspitsyn

src/java.base/share/classes/java/lang/ThreadGroup.java

@@ -792,6 +792,14 @@ public class ThreadGroup implements Thread.UncaughtExceptionHandler {
         }
     }
 
+    /**
+     * Returns a snapshot of the subgroups as an array, used by JVMTI.
+     */
+    private ThreadGroup[] subgroupsAsArray() {
+        List<ThreadGroup> groups = synchronizedSubgroups();
+        return groups.toArray(new ThreadGroup[0]);
+    }
+
     /**
      * Returns a snapshot of the subgroups.
      */