Auto merge of #20508 - nnethercote:bug-1448225, r=emilio

Convert StylePrefs to StaticPrefs.

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This is the Servo part of https://bugzilla.mozilla.org/show_bug.cgi?id=1448225.
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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 https://bugzilla.mozilla.org/show_bug.cgi?id=1448225

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- [ ] There are tests for these changes OR
- [X] These changes do not require tests because tested on the Gecko side.

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r? emilio

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