LibWeb: Fix CloseWatcher constructor for detached iframes

This fixes the last subtest in /close-watcher/frame-removal.html :)

Tests/LibWeb/Text/expected/HTML/CloseWatcher-detached-iframe.txt

@@ -0,0 +1 @@
+PASS

Tests/LibWeb/Text/input/HTML/CloseWatcher-detached-iframe.html

@@ -0,0 +1,25 @@
+<script src="../include.js"></script>
+<script>
+    promiseTest(async () => {
+        const iframe = document.createElement('iframe');
+        document.body.appendChild(iframe);
+
+        await new Promise(resolve => iframe.onload = resolve);
+
+        const iframeCloseWatcher = iframe.contentWindow.CloseWatcher;
+        const iframeDOMException = iframe.contentWindow.DOMException;
+
+        iframe.remove();
+
+        try {
+            new iframeCloseWatcher();
+            println("FAIL");
+        } catch (error) {
+            if (error instanceof iframeDOMException && error.name === "InvalidStateError") {
+                println("PASS");
+            } else {
+                println(`FAIL: CloseWatcher construction threw unexpected error: ${error.name}`);
+            }
+        }
+    });
+</script>

Userland/Libraries/LibWeb/HTML/CloseWatcher.cpp

@@ -13,6 +13,7 @@
 #include <LibWeb/HTML/CloseWatcher.h>
 #include <LibWeb/HTML/CloseWatcherManager.h>
 #include <LibWeb/HTML/EventHandler.h>
+#include <LibWeb/HTML/HTMLIFrameElement.h>
 #include <LibWeb/HTML/Window.h>
 
 namespace Web::HTML {
@@ -41,9 +42,14 @@ JS::NonnullGCPtr<CloseWatcher> CloseWatcher::establish(HTML::Window& window)
 // https://html.spec.whatwg.org/multipage/interaction.html#dom-closewatcher
 WebIDL::ExceptionOr<JS::NonnullGCPtr<CloseWatcher>> CloseWatcher::construct_impl(JS::Realm& realm, CloseWatcherOptions const& options)
 {
-    // 1. If this's relevant global object's associated Document is not fully active, then return an "InvalidStateError" DOMException.
-    // FIXME: Not in spec explicitly, but this should account for detached iframes too. See /close-watcher/frame-removal.html WPT.
     auto& window = verify_cast<HTML::Window>(realm.global_object());
+
+    // NOTE: Not in spec explicitly, but this should account for detached iframes too. See /close-watcher/frame-removal.html WPT.
+    auto navigable = window.navigable();
+    if (navigable && navigable->has_been_destroyed())
+        return WebIDL::InvalidStateError::create(realm, "The iframe has been detached"_string);
+
+    // 1. If this's relevant global object's associated Document is not fully active, then return an "InvalidStateError" DOMException.
     if (!window.associated_document().is_fully_active())
         return WebIDL::InvalidStateError::create(realm, "The document is not fully active."_string);