Bug 1588883 - Restore fullscreen transition message. r=mconley

Differential Revision: https://phabricator.services.mozilla.com/D49373

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

browser/actors/DOMFullscreenParent.jsm

@@ -22,7 +22,7 @@ class DOMFullscreenParent extends JSWindowActorParent {
       }
       case "DOMFullscreen:NewOrigin": {
         // Don't show the warning if we've already exited fullscreen.
-        if (window.fullscreen) {
+        if (window.document.fullscreen) {
           window.PointerlockFsWarning.showFullScreen(
             aMessage.data.originNoSuffix
           );