HTML: add test for history.state after document.open() (#12650)

For whatwg/html#3946.

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

@@ -0,0 +1,29 @@
+async_test(t => {
+  const iframe = document.body.appendChild(document.createElement("iframe"));
+  t.add_cleanup(() => iframe.remove());
+  iframe.src = "/common/blank.html";
+  iframe.onload = t.step_func_done(() => {
+    const win = iframe.contentWindow;
+    const doc = iframe.contentDocument;
+    assert_equals(win.history.state, null);
+    win.history.replaceState("state", "");
+    assert_equals(win.history.state, "state");
+    assert_equals(doc.open(), doc);
+    assert_equals(win.history.state, "state");
+  });
+}, "history.state is kept by document.open()");
+
+async_test(t => {
+  const iframe = document.body.appendChild(document.createElement("iframe"));
+  t.add_cleanup(() => iframe.remove());
+  iframe.src = "/common/blank.html";
+  iframe.onload = t.step_func_done(() => {
+    const win = iframe.contentWindow;
+    const doc = iframe.contentDocument;
+    assert_equals(win.history.state, null);
+    win.history.replaceState("state", "");
+    assert_equals(win.history.state, "state");
+    assert_equals(doc.open("", "replace"), doc);
+    assert_equals(win.history.state, "state");
+  });
+}, "history.state is kept by document.open() (with historical replace parameter set)");