Fix interfaces/Document/getAnimations.html test

This test was assuming that the iframe load event could not fire during
appendChild, which it does on Chrome. To fix this, setup the promise
before calling appendChild, but only await on it afterwards.

Bug: None
Change-Id: Ie5ebdd7cec0fe0c276621033afaa8e8a5699dad1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1833902Reviewed-by: Kevin Ellis <kevers@chromium.org>
Commit-Queue: Stephen McGruer <smcgruer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#702037}

third_party/blink/web_tests/external/wpt/web-animations/interfaces/Document/getAnimations-expected.txt

@@ -4,7 +4,7 @@ PASS Test document.getAnimations for script-generated animations
 PASS Test the order of document.getAnimations with script generated animations
 PASS Test document.getAnimations for a disconnected node
 PASS Test document.getAnimations with null target
-FAIL Test document.getAnimations for elements inside same-origin iframes assert_true: Not expecting event, but got load event expected true got false
+FAIL Test document.getAnimations for elements inside same-origin iframes assert_equals: expected 1 but got 0
 PASS Triggers a style change event
 Harness: the test ran to completion.
 

third_party/blink/web_tests/external/wpt/web-animations/interfaces/Document/getAnimations.html

@@ -68,11 +68,12 @@ promise_test(async t => {
   const iframe = document.createElement('iframe');
 
   const eventWatcher = new EventWatcher(t, iframe, ['load']);
+  const event_promise = eventWatcher.wait_for('load');
 
   document.body.appendChild(iframe);
   t.add_cleanup(() => { document.body.removeChild(iframe); });
 
-  await eventWatcher.wait_for('load');
+  await event_promise;
 
   const div = createDiv(t, iframe.contentDocument)
   const effect = new KeyframeEffect(div, null, 100 * MS_PER_SEC);