Don't use std::result_of am: 2d1fbf685c

Original change: https://android-review.googlesource.com/c/platform/external/libtextclassifier/+/2901832

Change-Id: I9940fc80a349d512720dd978c28176353ee85d1d
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>