Work around flaky TextTrackList tests that will be stable with Oilpan

LayoutTests/media/track/opera/interfaces/TextTrackList/onaddtrack.html

Index: LayoutTests/media/track/opera/interfaces/TextTrackList/onaddtrack.html
diff --git a/LayoutTests/media/track/opera/interfaces/TextTrackList/onaddtrack.html b/LayoutTests/media/track/opera/interfaces/TextTrackList/onaddtrack.html
index 6783aee264c44128f6dd1698615e40c55dec689f..7c0a8a9d1e7176dc70c20c1dc9a3b5db4f521031 100644
--- a/LayoutTests/media/track/opera/interfaces/TextTrackList/onaddtrack.html
+++ b/LayoutTests/media/track/opera/interfaces/TextTrackList/onaddtrack.html
@@ -5,7 +5,10 @@
 <div id=log></div>
 <script>
 setup(function(){
-    window.tracks = document.createElement('video').textTracks;
+    // TODO(philipj): Remove this test in favor of those in web-platform-tests
+    // once Oilpan has shipped. https://crbug.com/503852
+    window.video = document.createElement('video');
+    window.tracks = video.textTracks;
     window.ev = new Event('addtrack');
     window.ran = false;
     window.cb = function() { ran = true; };