RFR: 8365922: Parallel: Group uses of GCTimeRatio to a single location

[Paul Hohensee]

On Thu, 21 Aug 2025 10:55:23 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

> Unify all accesses of `GCTimeRatio` to a single place so that Parallel gets a coherence view of its value within the pause, if the global variable's value is ever changed.
> 
> Test: tier1-3

Marked as reviewed by phh (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/26881#pullrequestreview-3145207111