Bug 1594738 Remove top black line from kiosk mode; r=mkaply

There is a fullscr-toggler still shown in kiosk mode. This patch
disables it showing when in kiosk mode.

Differential Revision: https://phabricator.services.mozilla.com/D81828

browser/base/content/browser-fullScreenAndPointerLock.js

@@ -724,7 +724,9 @@ var FullScreen = {
       return;
     }
 
-    this._fullScrToggler.hidden = false;
+    if (!BrowserHandler.kiosk) {
+      this._fullScrToggler.hidden = false;
+    }
 
     if (
       aAnimate &&