[css-values] "fetch a style resource" should fall back to the CSSStyleSheet's location

In step 2 of ["fetch a style resource"]() we have:

> Let base be sheet’s [stylesheet base URL](https://drafts.csswg.org/cssom-1/#concept-css-style-sheet-stylesheet-base-url) if it is not null, otherwise environmentSettings’s [API base URL](https://html.spec.whatwg.org/multipage/webappapis.html#api-base-url). [[CSSOM]](https://drafts.csswg.org/css-values-4/#biblio-cssom)

However, only stylesheets constructed from JS have a base URL set. For those that don't, I believe we should fall back to sheet's [location](https://drafts.csswg.org/cssom-1/#concept-css-style-sheet-location), and only afterwards use the API base URL. As an example:

## index.html
```html
<!DOCTYPE html>
<style>
     @import url("data/sheet.css");
</style>
<div id="checkerboard"></div>
```

## data/sheet.css
```css
#checkerboard {
    background: url("image.png");
}
```

With the current algorithm, sheet has no base URL, so it would fall back to the API base URL, which is the document's location. It would thus try to load `image.png` instead of `data/image.png`. Inserting the sheet's location as a fallback means that it would load relative to `data/sheet.css`, and so get `data/image.png`.

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[css-values] "fetch a style resource" should fall back to the CSSStyleSheet's location #12068

index.html

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[css-values] "fetch a style resource" should fall back to the CSSStyleSheet's location #12068

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index.html

data/sheet.css

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