Bug 1509378 - Make test_input_event.html refer aEvent in checkIfInputIsEvent() rather than window.event r=smaug

This is an idiot mistake.  It refers window.event accidentally and it's still
disabled on Beta and Release channel.  Therefore, we should make it refer
aEvent instead.

On the other hand, it might be better to make our lint check whether test
refers window.event directly or not because it may check odd result
accidentally.

Differential Revision: https://phabricator.services.mozilla.com/D12870

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

dom/html/test/forms/test_input_event.html

@@ -57,7 +57,7 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=851780
   }
 
   function checkIfInputIsEvent(aEvent, aDescription) {
-    ok(event instanceof Event && !(event instanceof UIEvent),
+    ok(aEvent instanceof Event && !(aEvent instanceof UIEvent),
        `"input" event should be dispatched with InputEvent interface ${aDescription}`);
     is(aEvent.cancelable, false,
        `"input" event should be never cancelable ${aDescription}`);