RFR: 8307395: Add missing STS to Shenandoah

[Roman Kennke]

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

Testing (all in Lilliput where it caused the troubles, but applies to upstream as well):
 - [x] TestGCBasherWithShenandoah.java +UseHeavyMonitors
 - [x] hotspot_gc_shenandoah +UseHeavyMonitors

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Commit messages:
 - 8307395: Add missing STS to Shenandoah

Changes: https://git.openjdk.org/jdk/pull/13799/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13799&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/jdk/pull/13799.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13799/head:pull/13799

PR: https://git.openjdk.org/jdk/pull/13799