RFR: 8362278: G1: Consolidate functions for recording pause start time

[Ivan Walulya]

Hi,

Please review this refactoring change to unify the recording of GC Pause start-time.

Testing: Tier 1

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Commit messages:
 - clean out
 - init

Changes: https://git.openjdk.org/jdk/pull/26323/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26323&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8362278
  Stats: 41 lines in 3 files changed: 9 ins; 23 del; 9 mod
  Patch: https://git.openjdk.org/jdk/pull/26323.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26323/head:pull/26323

PR: https://git.openjdk.org/jdk/pull/26323