Fix a flaky html/dom/render-blocking WPT

This patch fixes a flaky WPT in html/dom/render-blocking/ that:
- LoadObserver used to check whether `target instanceof EventTarget`,
  which doesn't work if `target` is in a subframe. This patch changes
  it to check whether it has `addEventListener` method.
- The autofocus target used to be declared directly with markup, and
  as a result, it may be autofocused before `DOMContentLoaded` event,
  then the event listener added after DCL can't trigger. So this patch
  changes the element to be dynamically inserted after DCL.
- The last promise test is supposed to be run after the font finishes
  loading, so this test adds a missing await for that

Fixed: 1313014
Change-Id: Ia55667a1876f76eb0bb4f91543f2c1e9f2f4a8c0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3575243
Reviewed-by: Joey Arhar <jarhar@chromium.org>
Commit-Queue: Xiaocheng Hu <xiaochengh@chromium.org>
Cr-Commit-Position: refs/heads/main@{#990135}

third_party/blink/web_tests/external/wpt/html/dom/render-blocking/render-blocked-apis-by-preload-link.tentative.html

@@ -51,10 +51,18 @@
       });
     }
 
-    autofocusTest = rejectIfEventsFired(
-        preloadObserver.load,
-        iframe.contentDocument.getElementById('autofocus-target'),
-        'focus');
+    // The autofocus target must be created and inserted after subframe DCL so
+    // that we can add the event listener here before the subframe flushes
+    // autofocus candidates.
+    function createAutofocusTest(document) {
+      const target = document.createElement('textarea');
+      target.autofocus = 'autofocus';
+      const test = rejectIfEventsFired(preloadObserver.load, target, 'focus');
+      document.body.appendChild(target);
+      return test;
+    }
+    autofocusTest = createAutofocusTest(iframe.contentDocument);
+
     scrollTest = rejectIfEventsFired(
         preloadObserver.load,
         iframe.contentDocument.getElementById('scroll-target'),
@@ -115,6 +123,7 @@
     () => intersectionObserverTest,
     'Should not run the update intersection observers step when render-blocked');
 promise_test(async () => {
+  await iframe.contentDocument.fonts.ready;
   const target = iframe.contentDocument.getElementById('target');
   assert_equals(target.offsetHeight, 20);
   assert_equals(target.offsetWidth, 220);

third_party/blink/web_tests/external/wpt/html/dom/render-blocking/support/subframe-render-blocking-preload.html

@@ -11,8 +11,6 @@
 </style>
 <span id="target" style="font: 20px/1 custom-font">Lorem ipsum</span>
 
-<textarea id="autofocus-target" autofocus></textarea>
-
 <div id="scroll-target" style="height: 100px; overflow: scroll">
   <div style="height: 200px"></div>
 </div>
@@ -25,6 +23,12 @@
 </style>
 <div id="animation-target" style="height: 50px; animation: anim 100ms"></div>
 
+<!-- We will use a script-inserted element to verify if the browser flushes
+  -- autofocus candidates. If we insert the element here via markup, it may
+  -- be focused before the subframe DOMContentLoaded event, giving the parent
+  -- frame no opportunity to add a 'focus' event handler.
+-->
+
 <!-- We should also verify that the context lost steps for canvas are not run,
   -- but there's currently no way to reliably trigger a context lost in WPT.
   -- See https://github.com/web-platform-tests/wpt/issues/30039

third_party/blink/web_tests/external/wpt/html/dom/render-blocking/support/test-render-blocking.js

@@ -5,12 +5,12 @@ class LoadObserver {
   constructor(target) {
     this.finishTime = null;
     this.load = new Promise((resolve, reject) => {
-      if (target instanceof EventTarget) {
-        target.onload = ev => {
+      if (target.addEventListener) {
+        target.addEventListener('load', ev => {
           this.finishTime = ev.timeStamp;
           resolve(ev);
-        };
-        target.onerror = reject;
+        });
+        target.addEventListener('error', reject);
       } else if (typeof target === 'string') {
         const observer = new PerformanceObserver(() => {
           if (numberOfResourceTimingEntries(target)) {