HTML: document.open() and the unload event

For whatwg/html#3651.

html/webappapis/dynamic-markup-insertion/opening-the-input-stream/unload.window.js

@@ -0,0 +1,13 @@
+async_test(t => {
+  const frame = document.body.appendChild(document.createElement("iframe"));
+  frame.src = "/common/blank.html";
+  frame.onload = t.step_func_done(() => {
+    let happened = 0;
+    frame.contentWindow.onpagehide = frame.contentWindow.onunload = t.step_func(() => {
+      happened++;
+    });
+    frame.contentDocument.open();
+    assert_equals(happened, 2);
+    frame.contentDocument.close();
+  });
+}, "document.open() and the unload event");