Make HTMLMediaElement.setMediaKeys() asynchronous.

LayoutTests/media/encrypted-media/encrypted-media-lifetime-reload.html

Index: LayoutTests/media/encrypted-media/encrypted-media-lifetime-reload.html
diff --git a/LayoutTests/media/encrypted-media/encrypted-media-lifetime-reload.html b/LayoutTests/media/encrypted-media/encrypted-media-lifetime-reload.html
index 755ad56e926bcb5c237593b41a6f12f26f46e12d..9005f3795885f59d4fc95459a0a86e2e1006eec1 100644
--- a/LayoutTests/media/encrypted-media/encrypted-media-lifetime-reload.html
+++ b/LayoutTests/media/encrypted-media/encrypted-media-lifetime-reload.html
@@ -78,12 +78,11 @@
                 }
 
                 MediaKeys.create('org.w3.clearkey').then(function(mediaKeys) {
-
                     waitForEventAndRunStep('needkey', video, onNeedKey, test);
                     waitForEventAndRunStep('playing', video, onPlaying, test);
-
-                    video.setMediaKeys(mediaKeys);
                     video.src = '../content/test-encrypted.webm';
+                    return video.setMediaKeys(mediaKeys);
+                }).then(function(result) {
                     video.play();
                 }).catch(function(error) {
                     forceTestFailureFromPromise(test, error);