Fix http/tests/event-timing/event-timing-onloadthenobserve.html

Changes this test to properly wait for the first synthetic click
event to fire before proceeding with the rest of the test and validating
its expectations.

Bug: 872081
Change-Id: I0954ef7deff8415600effbc79cca7402a1b51583
Reviewed-on: https://chromium-review.googlesource.com/1170173
Commit-Queue: Ken Rockot <rockot@chromium.org>
Reviewed-by: Nicolás Peña Moreno <npm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582184}

third_party/WebKit/LayoutTests/http/tests/event-timing/event-timing-onloadthenobserve.html

@@ -59,7 +59,7 @@ registration are lost
   */
   async_test(function(t) {
       on_event(window, 'load', () => {
-        clickAndBlockMain('button').then(wait).then(() => {
+        clickAndBlockMain('button').then(() => {
           startObserver(t);
           clickAndBlockMain('button').then(wait);
           processingStartMin = performance.now();

third_party/WebKit/LayoutTests/http/tests/event-timing/resources/event-timing-support.js

-function clickOnElement(id) {
-  const rect = document.getElementById(id).getBoundingClientRect();
+function clickOnElement(id, callback) {
+  const element = document.getElementById(id);
+  const rect = element.getBoundingClientRect();
   const xCenter = rect.x + rect.width / 2;
   const yCenter = rect.y + rect.height / 2;
   var pointerActions = [{
@@ -9,6 +10,12 @@ function clickOnElement(id) {
       { name: "pointerUp" },
     ]
   }];
+  var clickHandler = () => {
+    if (callback)
+      callback();
+    element.removeEventListener("click", clickHandler);
+  };
+  element.addEventListener("click", clickHandler);
   if (!chrome || !chrome.gpuBenchmarking) {
     reject();
   } else {
@@ -60,8 +67,7 @@ function wait() {
 
 function clickAndBlockMain(id) {
   return new Promise((resolve, reject) => {
-    clickOnElement(id);
+    clickOnElement(id, resolve);
     mainThreadBusy(300);
-    resolve();
   });
 }