We sometimes store function indexes and number of functions as {size_t}
and sometimes as {int}. Unify a few places to be {int}.

R=ahaas@chromium.org

Change-Id: I1d204cbd9388245f97f291a469b32743457ab2c0
Reviewed-on: https://chromium-review.googlesource.com/c/1491607Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59946}