RFR: 8314990: Generational ZGC: Strong OopStorage stats reported as weak roots

[Jonas Norlinder]

`ZOopStorageSetIteratorStrong::apply` has incorrect `ZRootStatTimer`event. Should record strong roots but are recording weak roots.

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Commit messages:
 - Fix ZRootStatTimer event

Changes: https://git.openjdk.org/jdk/pull/15420/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15420&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8314990
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/15420.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15420/head:pull/15420

PR: https://git.openjdk.org/jdk/pull/15420