Improve reliability of EME tests

LayoutTests/media/encrypted-media/encrypted-media-waiting-for-a-key.html

Index: LayoutTests/media/encrypted-media/encrypted-media-waiting-for-a-key.html
diff --git a/LayoutTests/media/encrypted-media/encrypted-media-waiting-for-a-key.html b/LayoutTests/media/encrypted-media/encrypted-media-waiting-for-a-key.html
index a94f4a4e350aa8c3f05540e9da00b01cb165b239..e09acd65d5f12ab0b80050fec13eb18ec50dea61 100644
--- a/LayoutTests/media/encrypted-media/encrypted-media-waiting-for-a-key.html
+++ b/LayoutTests/media/encrypted-media/encrypted-media-waiting-for-a-key.html
@@ -14,8 +14,8 @@
             {
                 var video = document.getElementById('testVideo');
                 var mediaKeySession = null;
-                var sessionCreated = false;
                 var sessionReadyReceived = false;
+                var encryptedEventCount = 0;
 
                 var keyId = null;
                 var rawKey = new Uint8Array([0xeb, 0xdd, 0x62, 0xf1, 0x68, 0x14, 0xd2, 0x7b,
@@ -28,10 +28,12 @@
                     assert_equals(event.type, 'encrypted');
 
                     // The same decryption key is shared by all streams so only
-                    // create a shared session once.
-                    if (sessionCreated)
+                    // 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;
-                    sessionCreated = true;
 
                     mediaKeySession = video.mediaKeys.createSession();
                     waitForEventAndRunStep('message', mediaKeySession, onMessage, test);