Although this seems to compile and pass on our bots, the std::pair
constructor with arguments is not constexpr until c++14. Instead
use a helper struct which achieves the same goal.

BUG=chromium:697758

Change-Id: I0f9873729485a5059f79af969cb56f84706e6c98
Reviewed-on: https://chromium-review.googlesource.com/545796Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>