Fix external/wpt/event-timing/event-timing-onloadobserve.html to be more deterministic

This test passes as some events like click or mousedown for the first
click are missed at the second PerformanceObserver in startObserver.
This CL makes sure that the first PerformanceObserver does not proceed
until the first click event happens.

Actually we found the test failed when we use a different task runner is
used for WidgetInputHandler (https://chromium-review.googlesource.com/c/chromium/src/+/1530492/4).

Bug: 872081
Change-Id: I8d8fe187b8053140f40ae9cf08737fcf7b64233c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1538136
Commit-Queue: Nicolás Peña Moreno <npm@chromium.org>
Reviewed-by: Nicolás Peña Moreno <npm@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#644321}

third_party/blink/web_tests/external/wpt/event-timing/event-timing-onloadthenobserve.html

@@ -66,8 +66,10 @@ registration are lost
     // Use a dummy observer to know when the first click has been dispatched.
     const observerPromise = new Promise((resolve, reject) => {
       new PerformanceObserver((entryList, observer) => {
-        resolve();
-        observer.disconnect();
+        if (entryList.getEntries().find(entry => entry.name === 'click')) {
+          resolve();
+          observer.disconnect();
+        }
       }).observe({ entryTypes: ['event'] });
     });
     on_event(window, 'load', () => {