Auto merge of #21045 - jonathanKingston:make-parent-replaceable, r=jdm

Allow window.parent attribute to be replaceable by JavaScript.

Permit JS to override the window.parent attribute.

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- [x] `./mach test-tidy` does not report any errors
- [X] These changes fix #20932 (github issue number if applicable).
- [x] There are tests for these changes OR

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tests/wpt/metadata/html/browsers/the-window-object/window-properties.https.html.ini

@@ -50,9 +50,6 @@
   [Window replaceable attribute: toolbar]
     expected: FAIL
 
-  [Window replaceable attribute: parent]
-    expected: FAIL
-
   [Window replaceable attribute: external]
     expected: FAIL