Implement setter for document.domain
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This PR implements the setter for `document.domain`.
It builds on #15438 and #15478, only the last commit is part of this PR.
It includes tests for similar-origin security checks.
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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix#934.
- [X] There are tests for these changes.
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Fix check to set flag if any header is a simple header.
Fix failing tests to allow merge in advance of @nikkibee's changes
as per @jdm's direction via IRC
