Auto merge of #28635 - negator:patch-2, r=jdm

Do not use ParallelIterator if not using rayon

The rayon [`ParalllelIIterator`](https://github.com/rayon-rs/rayon/blob/master/src/iter/plumbing/README.md) is being used for inline content size calculations irrespective of the `use_rayon` flag, which can lead to an unbounded (dynamically determined) number of threads being spawned during iteration.

The check is performed for static position calculations, however:
https://github.com/servo/servo/blob/6fced22e4784714540a2bf31b00c5f8149cbba87/components/layout_2020/flow/mod.rs#L235-L242

---
<!-- 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
- [ ] These changes fix #___ (GitHub issue number if applicable)

<!-- Either: -->
- [ ] There are tests for these changes OR
- [ ] These changes do not require tests because ___

<!-- Also, please make sure that "Allow edits from maintainers" checkbox is checked, so that we can help you if you get stuck somewhere along the way.-->

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

0 files changed, 0 insertions, 0 deletions