Add back CORS headers

Summary:
this seems accidently removed in this diff: D69179107

adding it back

Reviewed By: jai1623

Differential Revision: D70746795

fbshipit-source-id: 3c80115f679c8286620e40775e9436bf7efee0b4

Author: Ziyi Hao

facebook-for-wordpress.php

@@ -125,6 +125,12 @@ public function handle_events_request() {
                 $data
             );
         }
+        if ( isset( $_SERVER['HTTP_ORIGIN'] ) ) {
+            $origin = wp_kses( wp_unslash( $_SERVER['HTTP_ORIGIN'] ), array() );
+            header( "Access-Control-Allow-Origin: $origin" );
+            header( 'Access-Control-Allow-Credentials: true' );
+            header( 'Access-Control-Max-Age: 86400' );
+        }
         exit();
         }
     }