[core] Refactor getScrollbarSize to operate on window instead of document

[Janpot]

Change the implementation of getScrollbarSize to avoid the need to use non-null assertion. See #43818 (comment)

export default function getScrollbarSize(win: Window = window): number {
  // https://developer.mozilla.org/en-US/docs/Web/API/Window/innerWidth#usage_notes
  const documentWidth = win.document.documentElement.clientWidth;
  return Math.abs(win.innerWidth - documentWidth);
}

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[BrianWoolfolk]

Hi! I would like to work on this issue.
I just saw the PR and related issues and I'll be happy to change the implementations for this.

I also wanted to ask if this could be considered a good-first-issue? It seems straightforward-enough to me with all this given context

[BrianWoolfolk]

I've made a PR regarding this issue, you can see it here: #43828.

[oliviertassinari]

In mui/base-ui#604 (comment) I was wondering why there is an abs() in the first place, feels like something that we can remove.

On Base UI, for the next version of the Modal, they are going without a padding approach, so don't need getScrollbarSize() anymore.

[github-actions]

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