RFR: 8310354: G1: Annotate G1MMUTracker::when_sec with const

[Albert Mingkun Yang]

Simply adding `const` to G1MMUTracker APIs.

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Commit messages:
 - g1-mmu-const

Changes: https://git.openjdk.org/jdk/pull/14554/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14554&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8310354
  Stats: 8 lines in 2 files changed: 0 ins; 0 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/14554.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14554/head:pull/14554

PR: https://git.openjdk.org/jdk/pull/14554