Bug 1481400 - More explicitly activate checkbox in browser_protocol_ask_dialog_permission.js.

The element used to only receive the keyup event because of the
subdialog code moving around the focus.

MANUAL PUSH: Orange on a CLOSED TREE

uriloader/exthandler/tests/mochitest/browser_protocol_ask_dialog_permission.js

@@ -331,8 +331,7 @@ async function testCheckbox(
   }
 
   if (typeof actionCheckbox == "boolean") {
-    checkbox.focus();
-    await EventUtils.synthesizeKey("VK_SPACE", undefined, dialogEl.ownerWindow);
+    checkbox.click();
   }
 }