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- Fix #includes / remove unused ones.
- Fix some lint errors.
- Fix nits and use more constexpr.

Change-Id: I73af034b6855b4b7e4456aee6c1ae544cc7216b0
Reviewed-on: https://chromium-review.googlesource.com/1250641
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Chris Palmer <palmer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595296}