Include <functional> in headers that reference struct hash.

IWYU. This is more of https://crrev.com/c/1536740

TBR=bbudge@chromium.org,blundell@chromium.org,chcunningham@chromium.org,msw@chromium.org,reveman@chromium.org,rockot@google.com

Change-Id: I113c9c35ebab9b4d9d6b461d7fb0880a8e53d1e8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1536763
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Bill Budge <bbudge@chromium.org>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#645623}