Allow JSON messages to be returned for ClearKey.

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 e6ae2353f76f5bb0224a27c3f0a9ac2a79980d4b..095905e0db08bcab0d5eee1080a2affb6e556eb5 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
@@ -47,7 +47,7 @@
                     assert_equals(event.target, mediaKeySession);
                     assert_equals(event.type, 'message');
 
-                    keyId = new Uint8Array(event.message);
+                    keyId = extractFirstLicenseKey(event.message);
                     setTimeout(verifyWaitingForKey, 250);
                 }