Improve reliability of EME tests

LayoutTests/media/encrypted-media/encrypted-media-playback-setmediakeys-after-src.html

Index: LayoutTests/media/encrypted-media/encrypted-media-playback-setmediakeys-after-src.html
diff --git a/LayoutTests/media/encrypted-media/encrypted-media-playback-setmediakeys-after-src.html b/LayoutTests/media/encrypted-media/encrypted-media-playback-setmediakeys-after-src.html
index 4bfa79aba862ff86cebcce268b7775f779a9f9f6..3a95e4ad0a4a774e3c7e10d5a6cb6c4b052fd84a 100644
--- a/LayoutTests/media/encrypted-media/encrypted-media-playback-setmediakeys-after-src.html
+++ b/LayoutTests/media/encrypted-media/encrypted-media-playback-setmediakeys-after-src.html
@@ -14,8 +14,8 @@
             async_test(function(test)
             {
                 var video = document.getElementById('testVideo');
-                var isSessionCreated = false;
                 var isUpdatePromiseResolved = false;
+                var encryptedEventCount = 0;
 
                 var rawKey = new Uint8Array([0xeb, 0xdd, 0x62, 0xf1, 0x68, 0x14, 0xd2, 0x7b,
                                              0x68, 0xef, 0x12, 0x2a, 0xfc, 0xe4, 0xae, 0x3c]);
@@ -26,11 +26,13 @@
                     assert_true(event instanceof window.MediaEncryptedEvent);
                     assert_equals(event.type, 'encrypted');
 
-                    // The same decryption key is shared by all streams so
-                    // only create a shared session once.
-                    if (isSessionCreated)
+                    // The same decryption key is shared by all streams so only
+                    // create a shared session once. To avoid timing issues with
+                    // the "message" event being received before the second
+                    // "encrypted" event, only create the session on the second
+                    // event.
+                    if (++encryptedEventCount != 2)
                         return;
-                    isSessionCreated = true;
 
                     var mediaKeySession = video.mediaKeys.createSession();
                     waitForEventAndRunStep('message', mediaKeySession, onMessage, test);