Bug 1010206 - make the test webm-video/bug686957.html less likely to race when using async-video. r=padenot

layout/reftests/webm-video/bug686957.html

@@ -6,7 +6,7 @@
        preload="auto"
        id="v"
        onloadedmetadata="document.getElementById('v').currentTime=document.getElementById('v').duration"
-       onseeked="setTimeout(function(){document.documentElement.className = '';}, 0);"
+       onseeked="setTimeout(function(){document.documentElement.className = '';}, 15);"
 </video>
 </body>
 </html>