Make more of the Time-related class functions constexpr.

Bug: 761570
Change-Id: I69a8fe4e3aa7f5f573cd51435c44e0662f1579a3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1794269
Reviewed-by: Justin Schuh <jschuh@chromium.org>
Reviewed-by: Yuri Wiitala <miu@chromium.org>
Commit-Queue: Justin Schuh <jschuh@chromium.org>
Auto-Submit: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#696092}
3 files changed