RFR: 8297608: JFR: Incorrect duration after chunk rotation

[Erik Gahlin]

Could I have a review of PR that fixes a problem with the duration of instant events after a chunk rotation.

Testing: test/jdk/dk/jdk/jfr

Thanks
Erik

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Commit messages:
 - Update comment
 - Update comment
 - Initial

Changes: https://git.openjdk.org/jdk/pull/11364/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11364&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8297608
  Stats: 87 lines in 2 files changed: 78 ins; 5 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/11364.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11364/head:pull/11364

PR: https://git.openjdk.org/jdk/pull/11364