use std::underlying_type

Change-Id: If05a8e99c5754859a2b04f040097eff3461a42e6
Reviewed-on: https://skia-review.googlesource.com/c/189220
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
2 files changed