The std::pair constructor isn't constexpr there.

BUG=

Change-Id: I7711185c6c991fd5793cc6c785e9905710660cd9
Reviewed-on: https://chromium-review.googlesource.com/723849Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>