script: Remove named window proxy map entry when destroying nested browsing context (#42344)

Leaving the entries sitting around both unnecessarily increases memory
usage and breaks code that looks up window proxies by name and reuses
names for iframes within the same page.

Testing: Newly-passing tests.
Fixes: #15258

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

components/script/dom/html/htmliframeelement.rs

@@ -448,7 +448,9 @@ impl HTMLIFrameElement {
         self.pending_pipeline_id.set(None);
         self.about_blank_pipeline_id.set(None);
         self.webview_id.set(None);
-        self.browsing_context_id.set(None);
+        if let Some(browsing_context_id) = self.browsing_context_id.take() {
+            self.script_window_proxies.remove(browsing_context_id)
+        }
     }
 
     pub(crate) fn update_pipeline_id(