Per this discussion on blink-dev:
https://groups.google.com/a/chromium.org/d/topic/blink-dev/Gq69pQyfgS4/discussion

Blink's include rules should be changed to allow access to all headers in base/numerics/.

Some work here is already underway in https://chromium-review.googlesource.com/1108976 , removing WTF's checked_numeric.h. Other work in https://chromium-review.googlesource.com/1109267 allows inclusion of base's safe_conversions.h . We should expose the rest of the headers and allow access to all of their symbols.