js: Reinstate a missing variable declaration

... and then decide there was no excuse for renaming the variable, so
now it has the same name it had before I started using
Window.requestAnimationFrame().

emccpre.js

@@ -90,6 +90,7 @@ var midpoint_test_str = "ABCDEFGHIKLMNOPRSTUVWXYZ0123456789";
 var midpoint_cache = [];
 
 // Variables used by js_activate_timer() and js_deactivate_timer().
+var timer = null;
 var timer_active = false;
 var timer_reference;