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

This reverts commit 250c950a.

Reason for revert: Timing out on all bots: https://test-results.appspot.com/dashboards/flakiness_dashboard.html#tests=external/wpt/event-timing/event-timing-onloadthenobserve.html&testType=webkit_layout_tests

Original change's description:
> 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}

TBR=hajimehoshi@chromium.org,altimin@chromium.org,npm@chromium.org

Change-Id: Ibb4e463906f5b8040e96c7e008e999c638630e79
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 872081
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1540085Reviewed-by: Peter Kasting <pkasting@chromium.org>
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#644443}

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

@@ -66,10 +66,8 @@ 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) => {
-        if (entryList.getEntries().find(entry => entry.name === 'click')) {
-          resolve();
-          observer.disconnect();
-        }
+        resolve();
+        observer.disconnect();
       }).observe({ entryTypes: ['event'] });
     });
     on_event(window, 'load', () => {