Summary:
Found with modernize-pass-by-value
Signed-off-by: Rosen Penev <rosenp@gmail.com>
Pull Request resolved: https://github.com/facebook/folly/pull/1299

Differential Revision: D19270977

Pulled By: yfeldblum

fbshipit-source-id: 77c33a3425376838fa3c7b04b166137eb74f63ba