simplify SkTFitsIn

Now that we're C++14, a constexpr SkTFitsIn() is a lot easier.
All the old tests still pass, and one new added.

Updated the comments a bit for correctness and readability.

Change-Id: I0f60e32e545fe4f2fb14e66a2bf25d562dbd680f
Reviewed-on: https://skia-review.googlesource.com/133831
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
2 files changed