[jdk25u] RFR: 8367378: GenShen: Missing timing stats when old mark buffers are flushed during final update refs

[William Kemper]

Clean backport. Only touches Shenandoah and is a trivial change at that.

-------------

Commit messages:
 - Backport 11df28916af4c2bfd69b2829c74e046ad953ee6c

Changes: https://git.openjdk.org/jdk25u/pull/196/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk25u&pr=196&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8367378
  Stats: 2 lines in 2 files changed: 2 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk25u/pull/196.diff
  Fetch: git fetch https://git.openjdk.org/jdk25u.git pull/196/head:pull/196

PR: https://git.openjdk.org/jdk25u/pull/196