When a closure does not use `$this`, it can be made `static` for improved performance.
Static closures are supported in PHP since PHP 5.4.
Props jrf, hellofromTonya, swissspidy, SergeyBiryukov.
See #53359.
git-svn-id: https://develop.svn.wordpress.org/trunk@51657 602fd350-edb4-49c9-b593-d223f7449a82