Bug 1553678 - enable wpt 'track-active-cues.html' r=jya

According to the spec [1], the TimeMarchesOn should only be ran when media element's show poster flag is not set.

If we would like to receive `cuechange` event, we have to reset media element's `show poster` flag in order to run `TimeMarchesOn` when adding new cues.

[1] https://html.spec.whatwg.org/multipage/media.html#playing-the-media-resource:time-marches-on-2

Differential Revision: https://phabricator.services.mozilla.com/D32238

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

testing/web-platform/meta/html/semantics/embedded-content/media-elements/track/track-element/track-active-cues.html.ini

@@ -1,5 +0,0 @@
-[track-active-cues.html]
-  expected: TIMEOUT
-  [Ensure that no text track cues are active after the video is unloaded]
-    expected: TIMEOUT
-

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

@@ -17,6 +17,9 @@ async_test(function(t) {
 
     var video = document.createElement('video');
     video.src = getVideoURI('/media/movie_5');
+    // uanset media element's `show-poster` flag in order to run `time marches on`
+    // when we add new cues into media element's cues list.
+    video.play();
     var trackElement = document.createElement('track');
 
     trackElement.onload = t.step_func(eventCallback);