Auto merge of #17527 - cbrewster:iframe_cycles, r=asajeffrey

Don't return window proxy if it has been discarded

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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 #17479 (github issue number if applicable).

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tests/wpt/metadata/html/semantics/embedded-content/the-iframe-element/iframe-append-to-child-document.html.ini

@@ -1,6 +0,0 @@
-[iframe-append-to-child-document.html]
-  type: testharness
-  [Append iframe element to its own child document]
-    bug: https://github.com/servo/servo/issues/17479
-    expected: FAIL
-