Adds a new `Option<ReferrerPolicy>` field to Document and sets it appropriately in `ScriptThread::load` if a Referrer-Policy header is present.
r? @jdm
<!-- 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#11860
- [X] There are tests for these changes
<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->
Source-Repo: https://github.com/servo/servo
Source-Revision: b382cc2103180f7dfd8df9c34970a95ed57a2d88
6097b905a9