Fix LayoutTests that assume canplay, playing, and canplaythrough will only fire once.

LayoutTests/media/track/track-cues-pause-on-exit.html

Index: LayoutTests/media/track/track-cues-pause-on-exit.html
diff --git a/LayoutTests/media/track/track-cues-pause-on-exit.html b/LayoutTests/media/track/track-cues-pause-on-exit.html
index 0f650d703618c5723f363421881c78debb77fca1..d728d7c10ea2b8e99a081647fed69c2435d3a1f2 100644
--- a/LayoutTests/media/track/track-cues-pause-on-exit.html
+++ b/LayoutTests/media/track/track-cues-pause-on-exit.html
@@ -66,7 +66,7 @@
                 video.src = findMediaFile("video", "../content/test");
             }
 
-            waitForEvent('canplaythrough', function() {
+            waitForEventOnce('canplaythrough', function() {
                 video.currentTime = 4.00;
                 videoCanPlayThrough = true;