DOM: Add WPTs for removing steps

This CL adds WPTs for removing steps, to ensure that script is never
executed during the removing steps hook [1], which are invoked
synchronously during per-node removal (that is, they are not batched,
but run this way per-node).

This CL:
 1. Augments the existing iframe removing steps WPT, to include
    `pagehide` events. At the time of writing this, all browsers deviate
    from the HTML Standard, and fail this test.
 2. Adds new tests for `<input>` and `<button>` to assert that when they
    are focused and subsequently removed from the DOM, the `blur` event
    is *not* fired, per the HTML Standard. Chromium fails this test,
    while Safari and Firefox pass it.

[1]: https://dom.spec.whatwg.org/#concept-node-remove-ext

Bug: 40150299
Change-Id: I0aa43755ccac8ea432056800c5c4167d8e4460e8

dom/nodes/insertion-removing-steps/blur-event.window.js

@@ -0,0 +1,19 @@
+test(() => {
+  const input = document.body.appendChild(document.createElement('input'));
+  input.focus();
+
+  let blurCalled = false;
+  input.onblur = e => blurCalled = true;
+  input.remove();
+  assert_false(blurCalled, "Blur event was not fired");
+}, "<input> element does not fire blur event upon DOM removal");
+
+test(() => {
+  const button = document.body.appendChild(document.createElement('button'));
+  button.focus();
+
+  let blurCalled = false;
+  button.onblur = e => blurCalled = true;
+  button.remove();
+  assert_false(blurCalled, "Blur event was not fired");
+}, "<button> element does not fire blur event upon DOM removal");

dom/nodes/insertion-removing-steps/insertion-removing-steps-iframe.window.js

@@ -109,6 +109,15 @@ function runRemovalTest(removal_method) {
     t.add_cleanup(() => removalObserver.disconnect());
 
     let iframe1UnloadFired = false, iframe2UnloadFired = false;
+    let iframe1PagehideFired = false, iframe2PagehideFired = false;
+    iframe1.contentWindow.addEventListener('pagehide', e => {
+      assert_false(iframe1UnloadFired, "iframe1 pagehide fires before unload");
+      iframe1PagehideFired = true;
+    });
+    iframe2.contentWindow.addEventListener('pagehide', e => {
+      assert_false(iframe2UnloadFired, "iframe2 pagehide fires before unload");
+      iframe2PagehideFired = true;
+    });
     iframe1.contentWindow.addEventListener('unload', e => iframe1UnloadFired = true);
     iframe2.contentWindow.addEventListener('unload', e => iframe2UnloadFired = true);
 
@@ -126,6 +135,8 @@ function runRemovalTest(removal_method) {
       div.innerHTML = '';
     }
 
+    assert_false(iframe1PagehideFired, "iframe1 pagehide did not fire");
+    assert_false(iframe2PagehideFired, "iframe2 pagehide did not fire");
     assert_false(iframe1UnloadFired, "iframe1 unload did not fire");
     assert_false(iframe2UnloadFired, "iframe2 unload did not fire");