Bug 1550442 - check track's ready state in the beginning of the test. r=baku

If the track has already finished loading, we can directly run `trackLoaded()`.

Differential Revision: https://phabricator.services.mozilla.com/D37953

--HG--
extra : moz-landing-system : lando

testing/web-platform/tests/html/semantics/embedded-content/media-elements/track/track-element/track-mode.html

@@ -9,7 +9,13 @@
     async_test(function(t) {
         var video = document.querySelector("video");
         var track = document.querySelector("track");
-        track.onload = t.step_func(trackLoaded);
+        if (track.readyState != HTMLTrackElement.LOADED) {
+            assert_not_equals(track.readyState, HTMLTrackElement.ERROR,
+                              "track failed to load resource.");
+            track.onload = t.step_func(trackLoaded);
+        } else {
+            trackLoaded();
+        }
 
         var cueCount = 0;
         var textTrack;