Make SkTFitsIn and SkTo constexpr.

Because what can be better than marking things that can be constexpr as
constexpr. Also, I have a change where this would be nice to have.

Change-Id: Ie313b19d1930b98ddcd60cc17a320971625f18e3
Reviewed-on: https://skia-review.googlesource.com/60862
Commit-Queue: Ben Wagner <bungeman@google.com>
Reviewed-by: Mike Klein <mtklein@chromium.org>
2 files changed