Bug 1816233 - Clean up content-visibility-031.html. r=surkov

This makes assert failures getting reported properly rather than as
uncaught exceptions. No behavior change otherwise.

Differential Revision: https://phabricator.services.mozilla.com/D169543

testing/web-platform/tests/css/css-contain/content-visibility/content-visibility-031.html

@@ -20,10 +20,10 @@
 <script src="/resources/testharnessreport.js"></script>
 
 <script>
-async_test((t) => {
+async_test(t => {
   let didCallback = false;
 
-  async function runTest() {
+  function runTest() {
     let resizeCallback = function (entries) {
       didCallback = true;
     }
@@ -31,11 +31,11 @@ async_test((t) => {
 
     resizeObserver.observe(resize);
 
-    requestAnimationFrame(step2);
+    requestAnimationFrame(t.step_func(step2));
   }
 
-  async function step2() {
-    assert_equals(true, didCallback, 'Resize observation should happen in first frame after registering');
+  function step2() {
+    assert_true(didCallback, 'Resize observation should happen in first frame after registering');
     didCallback = false;
 
     const container = document.getElementById("container");
@@ -45,31 +45,27 @@ async_test((t) => {
     // only when the element becomes unhidden.
     resize.style.width = '100px';
 
-    requestAnimationFrame(step3);
+    requestAnimationFrame(t.step_func(step3));
   }
 
-  async function step3() {
-    assert_equals(false, didCallback,
-        'ResizeObsever should not run during while unrendered');
-    requestAnimationFrame(step4);
+  function step3() {
+    assert_false(didCallback, 'ResizeObsever should not run during while unrendered');
+    requestAnimationFrame(t.step_func(step4));
   }
 
-  async function step4() {
-    assert_equals(false, didCallback,
-        'ResizeObsever should not run while unrendered');
+  function step4() {
+    assert_false(didCallback, 'ResizeObsever should not run while unrendered');
     const container = document.getElementById("container");
     container.classList.remove("hidden");
-    requestAnimationFrame(step5);
+    requestAnimationFrame(t.step_func_done(step5));
   }
 
-  async function step5() {
-    assert_equals(true, didCallback,
-        'ResizeObsevers should now run once becoming visible');
-    t.done();
+  function step5() {
+    assert_true(didCallback, 'ResizeObsevers should now run once becoming visible');
   }
 
   window.onload = function() {
-    requestAnimationFrame(() => requestAnimationFrame(runTest));
+    requestAnimationFrame(() => requestAnimationFrame(t.step_func(runTest)));
   };
 }, "ResizeObserver skipped while hidden");
 </script>