Use associated types to improve LayoutThreadFactory and ScriptThreadFactory.
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 --faster` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).
Either:
- [ ] There are tests for these changes OR
- [x] These changes do not require tests because refactoring
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