Allow constexpr variables of TimeDelta type.

This allows TimeDelta variables to be global per https://google.github.io/styleguide/cppguide.html#Static_and_Global_Variables since they don't have to run a dynamic initializer anymore.

Review-Url: https://codereview.chromium.org/1976703005
Cr-Commit-Position: refs/heads/master@{#394527}
6 files changed