Fix onKeyAdded check causing flakiness in the tests.

content/test/data/media/encrypted_media_player.html

Index: content/test/data/media/encrypted_media_player.html
diff --git a/content/test/data/media/encrypted_media_player.html b/content/test/data/media/encrypted_media_player.html
index 8c7d0cd24b729f4bfc5bf04c97f00faa88c4f15b..bfff39bc2810274add2ae06e8dd66c8601a6412f 100644
--- a/content/test/data/media/encrypted_media_player.html
+++ b/content/test/data/media/encrypted_media_player.html
@@ -10,11 +10,12 @@
       var video = document.querySelector('video');
 
       function onTimeUpdate() {
-        // webkitkeyadded should have been fired before playback.
-        if (!video.receivedKeyAdded)
-          failTest('Key added event not received.');
         if (video.currentTime < 1)
           return;
+        // keyadded may be fired around the start of playback; check for it
+        // after a delay to avoid timing issues.
+        if (!video.receivedKeyAdded)
+          failTest('Key added event not received.');
         if (video.isHeartbeatExpected && !video.receivedHeartbeat)
           failTest('Heartbeat keymessage event not received.');
         video.removeEventListener('ended', failTest);