Stop iframes from including their ancestors
An iframe that included itself would just keep loading until running out of resources; the spec includes protection against that, and now Servo follows (some of) the spec about it.
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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 the same-origin case presented in #3931 but not a case in which an intermediate cross-origin iframe is hiding a same-origin ancestor further up.
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- [X] There is a manual test for these changes in #3931
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