Auto merge of #16920 - synlestidae:servo-16705, r=asajeffrey

Implement window.closed

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Implements #16705. window.closed should evaluate to true if BrowsingContext is discarded

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- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #16705 (github issue number if applicable).

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components/script/dom/webidls/Window.webidl

@@ -23,7 +23,7 @@
   //[Replaceable] readonly attribute BarProp toolbar;
   attribute DOMString status;
   void close();
-  //readonly attribute boolean closed;
+  readonly attribute boolean closed;
   //void stop();
   //void focus();
   //void blur();

components/script/dom/window.rs

@@ -509,6 +509,13 @@ impl WindowMethods for Window {
         }
     }
 
+    // https://html.spec.whatwg.org/multipage/#dom-window-closed
+    fn Closed(&self) -> bool {
+        self.window_proxy.get()
+            .map(|ref proxy| proxy.is_browsing_context_discarded())
+            .unwrap_or(true)
+    }
+
     // https://html.spec.whatwg.org/multipage/#dom-window-close
     fn Close(&self) {
         self.main_thread_script_chan()