Implement source for postMessage events
Also make similar-origin iframes access their newly loaded document as soon as the new document is created. This should allow tests that check the event.source property to run correctly.
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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#22499 and fix#12715 and fix#22514.
- [x] There are tests for these changes
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Update document.open to latest spec
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This is one of my first contributions, so I might need some direction cleaning it up -- I ran web-platform-tests locally, but the suite has several intermittent passes/failures for me. Thanks!
Few notes:
* I may have been wrong to eliminate all of the resets listed in what was formerly Step 18 (starts with `self.implementation.set(None);`). It's not clear to me that they're still needed or if so, what step they would fall under, but I didn't notice any web platform tests break as a result.
* <s>If I'm reading the spec right, there's a discrepancy in the error returned by the three-parameter overload of Document.open between the spec and web-platform-tests/implementations in other browsers. As written, I favored the spec, but it causes one web-platform-test to fail.</s> This has been resolved in https://github.com/whatwg/html/pull/4066
* I'm not 100% certain that tests pass as expected, I had several intermittent failures that disappeared when re-run.
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- [x] These changes fix#21445
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