[ticket/12843] Fix Mark forums read browser console error

When you click on "Mark forums read" in the browser console error occurs
ReferenceError: assignment to undeclared variable phpbbAlertTimer
ajax.js (line13)

PHPBB3-12843

phpBB/assets/javascript/core.js

@@ -47,6 +47,18 @@ phpbb.clearLoadingTimeout = function() {
 	}
 };
 
+
+/**
+* Close popup alert after a specified delay
+*
+* @param int Delay in ms until darkenwrapper's click event is triggered
+*/
+phpbb.closeDarkenWrapper = function(delay) {
+	phpbbAlertTimer = setTimeout(function() {
+		$('#darkenwrapper').trigger('click');
+	}, delay);
+};
+
 /**
  * Display a simple alert similar to JSs native alert().
  *