[master] RFR: 8307395: [Lilliput] Add missing STS to Shenandoah

[Roman Kennke]

Testing has revealed that Shenandoah GC is lacking one STS. This causes a reliable crash when running TestGCBasherWithShenandoah.java with -XX:+UseHeavyMonitors because it touches an already deflated monitor.

Testing:
 - [x] TestGCBasherWithShenandoah.java +UseHeavyMonitors
 - [x] hotspot_gc_shenandoah +UseHeavyMonitors

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Commit messages:
 - 8307395: [Lilliput] Add missing STS to Shenandoah

Changes: https://git.openjdk.org/lilliput/pull/90/files
 Webrev: https://webrevs.openjdk.org/?repo=lilliput&pr=90&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8307395
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/lilliput/pull/90.diff
  Fetch: git fetch https://git.openjdk.org/lilliput.git pull/90/head:pull/90

PR: https://git.openjdk.org/lilliput/pull/90