Bug 1680896 - Try to make test_pending_animation_tracker more reliable. r=birtles

I couldn't reproduce this intermittent, but this test is prone to
throttling decisions. In fact, without this check:

  https://searchfox.org/mozilla-central/rev/7751fef9eeb3db0a07ae4680daa2a62bd8f49882/dom/base/Document.cpp#7055

I did see consistent failures. Ensure the window is fully loaded so that
the refresh driver doesn't get throttled unexpectedly.

Try to enable the test across platforms.

Differential Revision: https://phabricator.services.mozilla.com/D147633

dom/animation/test/mochitest.ini

@@ -51,9 +51,6 @@ skip-if = (os == 'win' && bits == 64) # Bug 1363957
 [mozilla/test_mainthread_synchronization_pref.html]
 [mozilla/test_moz_prefixed_properties.html]
 [mozilla/test_pending_animation_tracker.html]
-skip-if =
-  fission && xorigin && os == "linux" && !debug  # Bug 1716403 - New fission platform triage
-  win10_2004 && !debug # Bug 1680896
 [mozilla/test_restyles.html]
 support-files =
   mozilla/file_restyles.html

dom/animation/test/mozilla/test_pending_animation_tracker.html

@@ -11,6 +11,12 @@
 <script>
 "use strict";
 
+promise_test(function waitForLoad() {
+  return new Promise(resolve => {
+    window.addEventListener("load", resolve, { once: true });
+  });
+});
+
 promise_test(async t => {
   // See below, but we should ensure we are in a rAF callback before proceeding
   // or else we will get inconsistent results.