Auto merge of #25456 - pshaughn:pastnamesexpire, r=jdm

Past names expire when form owner is reset

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On their way out of form.controls, elements now also leave form.past_names_map, passing one WPT test. We're already scanning linearly through form.controls linearly to get the index of the control there, so additionally scanning through the past names map by value shouldn't raise any particular performance concern.

---
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- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #25429

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- [X] There are tests for these changes

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components/script/dom/htmlformelement.rs

@@ -1081,6 +1081,13 @@ impl HTMLFormElement {
             .iter()
             .position(|c| &**c == control)
             .map(|idx| controls.remove(idx));
+
+        // https://html.spec.whatwg.org/multipage#forms.html#the-form-element:past-names-map-5
+        // "If an element listed in a form element's past names map
+        // changes form owner, then its entries must be removed
+        // from that map."
+        let mut past_names_map = self.past_names_map.borrow_mut();
+        past_names_map.retain(|_k, v| v.0 != control);
     }
 }