Instead of explicitly instantiating six copies of each function for
zero to five arguments, define them as variadic template.
This avoids the arbitrary limit to five arguments, and avoids lots of
code duplication.

Drive-by fix: [iwyu] Add missing include.

R=mstarzinger@chromium.org

Change-Id: Id7edde3611ddfade16c7afbbf4b9000cedd76468
Reviewed-on: https://chromium-review.googlesource.com/496247Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45130}