Fix usage of onload = t.step_timeout in WPT tests

t.step_timeout directly invokes setTimeout, so this pattern doesn't
actually wait for onload. The pattern, as used elsewhere, should be
something like:

  onload = () => t.step_timeout

One app-history test in particular was causing issues as it became
dependent timer task ordering.

Bug: 1220223
Change-Id: I2a8c74e485833a3bc15cc53320c791ecb80452af
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3499359
Auto-Submit: Scott Haseley <shaseley@chromium.org>
Reviewed-by: Nate Chapin <japhet@chromium.org>
Commit-Queue: Nate Chapin <japhet@chromium.org>
Cr-Commit-Position: refs/heads/main@{#976889}

app-history/currentchange-event/currentchange-app-history-back-forward-same-doc.html

@@ -5,7 +5,7 @@
 promise_test(async t => {
   // Wait for after the load event so that the navigation doesn't get converted
   // into a replace navigation.
-  await new Promise(resolve => window.onload = t.step_timeout(resolve, 0));
+  await new Promise(resolve => window.onload = () => t.step_timeout(resolve, 0));
   await appHistory.navigate("#foo").committed;
   assert_equals(appHistory.entries().length, 2);
 

app-history/currentchange-event/currentchange-app-history-navigate-replace-same-doc.html

@@ -5,7 +5,7 @@
 promise_test(async t => {
   // Wait for after the load event so that the navigation doesn't get converted
   // into a replace navigation.
-  await new Promise(resolve => window.onload = t.step_timeout(resolve, 0));
+  await new Promise(resolve => window.onload = () => t.step_timeout(resolve, 0));
 
   let oncurrentchange_called = false;
   let original_entry = appHistory.current;

app-history/currentchange-event/currentchange-app-history-navigate-same-doc.html

@@ -5,7 +5,7 @@
 promise_test(async t => {
   // Wait for after the load event so that the navigation doesn't get converted
   // into a replace navigation.
-  await new Promise(resolve => window.onload = t.step_timeout(resolve, 0));
+  await new Promise(resolve => window.onload = () => t.step_timeout(resolve, 0));
 
   let oncurrentchange_called = false;
   appHistory.oncurrentchange = t.step_func(e => {

app-history/currentchange-event/currentchange-history-back-same-doc.html

@@ -5,7 +5,7 @@
 promise_test(async t => {
   // Wait for after the load event so that the navigation doesn't get converted
   // into a replace navigation.
-  await new Promise(resolve => window.onload = t.step_timeout(resolve, 0));
+  await new Promise(resolve => window.onload = () => t.step_timeout(resolve, 0));
   await appHistory.navigate("#foo");
   assert_equals(appHistory.entries().length, 2);
 

app-history/navigate-event/navigate-history-back-after-fragment.html

@@ -3,7 +3,7 @@
 <script src="/resources/testharnessreport.js"></script>
 <script>
 async_test(t => {
-  window.onload = t.step_timeout(() => {
+  window.onload = () => t.step_timeout(() => {
     let start_length = history.length;
     let target_key = appHistory.current.key;
     let target_id = appHistory.current.id;

app-history/navigate-event/navigate-history-back-after-pushState.html

@@ -3,7 +3,7 @@
 <script src="/resources/testharnessreport.js"></script>
 <script>
 async_test(t => {
-  window.onload = t.step_timeout(() => {
+  window.onload = () => t.step_timeout(() => {
     let start_length = history.length;
     let target_key = appHistory.current.key;
     let target_id = appHistory.current.id;

html/semantics/forms/the-selectmenu-element/selectmenu-form-state-restore.tentative.html

@@ -16,7 +16,7 @@
 
 <script>
 async_test(t => {
-  window.onload = t.step_timeout(() => {
+  window.onload = () => t.step_timeout(() => {
     let state = document.getElementById('emptyOnFirstVisit');
     let selectMenu = document.getElementById("selectmenu0");