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

LayoutTests/media/video-duration-known-after-eos.html

Index: LayoutTests/media/video-duration-known-after-eos.html
diff --git a/LayoutTests/media/video-duration-known-after-eos.html b/LayoutTests/media/video-duration-known-after-eos.html
index d09d826f28852da8082212130f7dd76a7f15d835..7f38b51612ddcacb3fc7cffe22bb53e000f8fef8 100644
--- a/LayoutTests/media/video-duration-known-after-eos.html
+++ b/LayoutTests/media/video-duration-known-after-eos.html
@@ -16,6 +16,7 @@
 
             function playing()
             {
+                video.removeEventListener("playing", playing);
                 run("video.currentTime = video.duration - 0.2");
             }
 
@@ -43,5 +44,3 @@
         <p>Tests that duration is known after playback ended.</p>
     </body>
 </html>
-
-