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This PR implements `window.parent` and `window.top` for dissimilar-origin windows.
This PR builds on #15536, only the last commit is part of this PR.
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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#14996 and fix#11660.
- [X] These changes do not require tests because there's already a parentage test in `mozilla/cross-origin-objects/cross-origin-objects.html`.
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Source-Repo: https://github.com/servo/servo
Source-Revision: 0a3b373a6ae4fdd68ab9d2a7386733ab0bdebe4b