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This allows removing `#![feature(as_unsafe_cell)]` in geckolib and make progress towards #11815.
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<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [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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Source-Repo: https://github.com/servo/servo
Source-Revision: 3de14d9ca0cf54e6a2ae4eac5ff47e38e724b77b
899e94c60b