The stock GCC on Ubuntu 16.04 complains these constants
are unused (possibly gcc issue). This CL changes these
to constexpr to workaround gcc errors.

R=clemensh@chromium.org, joransiu@ca.ibm.com

Change-Id: I8c1772e91744bc46ace6bee576b90d40c0cdf41f
Reviewed-on: https://chromium-review.googlesource.com/881554Reviewed-by: Hannes Payer <hpayer@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Junliang Yan <jyan@ca.ibm.com>
Cr-Commit-Position: refs/heads/master@{#50936}