No bug - Add a bug number to the expected failure in message-event.html.

DONTBUILD

--HG--
extra : amend_source : 0473e70eeef44ce9632dacc40eee1dce31c983ce

testing/web-platform/meta/workers/interfaces/DedicatedWorkerGlobalScope/postMessage/message-event.html.ini

@@ -2,4 +2,5 @@
   type: testharness
   [Properties of the 'message' event]
     expected: FAIL
+    bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1164834