Base: Mark a few TimeClass methods as constexpr.

This will allow use of these methods in constexpr expressions.

Change-Id: If93c03a93aaa0dd6d4442f40ea296ebcfa80a862
Reviewed-on: https://chromium-review.googlesource.com/1219770


Reviewed-by: Mark Mentovai <mark@chromium.org>
Commit-Queue: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590480}