Switch to C++14 compatible statement
Switch to C++14 compatible statement

Summary: `std::map::extract` is C++17 only, but not all of the C++ support libraries provide it. At least for a little while longer.

Reviewed By: zhxchen17

Differential Revision: D25402680

fbshipit-source-id: fba02817598197813d0f2814606916393a7f1974

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