Summary:
On x86_64, this makes the struct 8 bytes smaller, so creating a PerfStepTimer on the stack will use slightly less stack space.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/7931
Reviewed By: jay-zhuang
Differential Revision: D26529470
Pulled By: ajkr
fbshipit-source-id: bbe2e843167152ffa05a5946f1add6621c9849f7