Bug 1848125 - Make MessageEvent a ProbablyShortLivingWrapper. r=smaug

It is expected that those objects are not held unto, this allows minor GCs to
usually collect the wrapper.

Differential Revision: https://phabricator.services.mozilla.com/D186435

dom/webidl/MessageEvent.webidl

@@ -7,7 +7,7 @@
  * https://html.spec.whatwg.org/#messageevent
  */
 
-[Exposed=(Window,Worker,AudioWorklet)]
+[Exposed=(Window,Worker,AudioWorklet), ProbablyShortLivingWrapper]
 interface MessageEvent : Event {
   constructor(DOMString type, optional MessageEventInit eventInitDict = {});