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] These changes fix #20932 (github issue number if applicable).
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components/script/dom/webidls/Window.webidl

@@ -38,7 +38,7 @@
   //         attribute any opener;
   // Note that this can return null in the case that the browsing context has been discarded.
   // https://github.com/whatwg/html/issues/2115
-  readonly attribute WindowProxy? parent;
+  [Replaceable] readonly attribute WindowProxy? parent;
   readonly attribute Element? frameElement;
   //WindowProxy open(optional DOMString url = "about:blank", optional DOMString target = "_blank",
   //                 optional DOMString features = "", optional boolean replace = false);