Auto merge of #17845 - asajeffrey:script-paint-worklet-background-size, r=glennw

Use CSS background-size property when computing the size of a paint worklet

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The size of a paint worklet should be based on the background-size CSS property.

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #17676.
- [X] These changes do not require tests because the existing css-paint-api tests catch this.

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