Fixes building in MSVS with -std:c++17

Refs: https://en.cppreference.com/w/cpp/utility/functional/unary_function
Change-Id: Ibb05b6810cae029aae33957f3eb52cc3a9e4863e
Reviewed-on: https://chromium-review.googlesource.com/c/1227322
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57428}