Auto merge of #11835 - servo:diy-as-unsafe-cell, r=nox

Use our copy of RefCell for style data

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This allows removing `#![feature(as_unsafe_cell)]` in geckolib and make progress towards #11815.

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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 part of #11815.

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- [ ] There are tests for these changes OR
- [x] These changes do not require new tests because they don’t introduce no functional changes.

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