MDL-46207 Tasks: Do not run tasks if their component is disabled.

This can be overridden on a per task basis by implementing:
get_run_if_component_disabled() in the scheduled task.

lib/classes/task/scheduled_task.php

@@ -183,6 +183,15 @@ abstract class scheduled_task extends task_base {
         return $this->disabled;
     }
 
+    /**
+     * Override this function if you want this scheduled task to run, even if the component is disabled.
+     *
+     * @return bool
+     */
+    public function get_run_if_component_disabled() {
+        return false;
+    }
+
     /**
      * Take a cron field definition and return an array of valid numbers with the range min-max.
      *