Since RTC_DCHECK was made constexpr compatible, we can now make the unit classes fully constexpr. Bug: webrtc:9883 Change-Id: I18973c2f318449869cf0bd45699c41be53fba806 Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/167722 Commit-Queue: Sebastian Jansson <srte@webrtc.org> Reviewed-by: Ali Tofigh <alito@webrtc.org> Cr-Commit-Position: refs/heads/master@{#30403}