RFR: 8333129: Move ShrinkHeapInSteps flag to Serial GC

[Zhengyu Gu]

On Wed, 29 May 2024 12:36:40 GMT, Zhengyu Gu <zgu at openjdk.org> wrote:

> A trivial change that moves Serial GC specific flag `ShrinkHeapInSteps` to `serial_globals.hpp`

I can confirm that `ShrinkHeapInSteps` flag is only used in `CardGeneration` and `CardGeneration` is only used in Serial and CMS in JDK9u source.

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PR Comment: https://git.openjdk.org/jdk/pull/19452#issuecomment-2139515483