GitLab

* Make more things constexpr and/or make it possible to do so later
* Inline SaturatedAdd/Sub(), FromDouble(), FromProduct()
* Order the FromUnits{,D}() pairs together
* Init members in declaration
* Briefer implementations of various functions
* IWYU
* Fix declared-but-not-defined issue for FromTimeSpec() w/OS_FUSCHIA
* Use more specific DCHECKs
* No else after return
* Omit needless qualifiers
* EXPECT -> static_assert where possible
* <atomic> is legal now
* Don't handle DCHECK failure

The inlines don't hurt size: this saves 4 KB off chrome.dll in my local
release build.

Bug: none
TBR: stevenjb@chromium.org
Change-Id: I269d6426ac1587569e7a4c785250a4dd5e95d5bf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2358997Reviewed-by: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Yuri Wiitala <miu@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#799459}