Bug 1584014, access PushService object to avoid leak r=lina

Differential Revision: https://phabricator.services.mozilla.com/D47189

--HG--
extra : moz-landing-system : lando

browser/components/BrowserGlue.jsm

@@ -2007,7 +2007,7 @@ BrowserGlue.prototype = {
 
     // Begin listening for incoming push messages.
     Services.tm.idleDispatchToMainThread(() => {
-      PushService.ensureReady();
+      PushService.wrappedJSObject.ensureReady();
     });
 
     Services.tm.idleDispatchToMainThread(() => {