Preserve coverage for unused functions by force marking them used when
code coverage is enabled.

Bug: chromium:927464
Change-Id: Ia973467d06f7268f4e98cc76d0bb98cc591e979c
Reviewed-on: https://chromium-review.googlesource.com/c/1454717
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59373}