add helper to check for overflow

pre-CL to aid in changing the convention for when we overflow

Bug: skia:
Change-Id: I1e34a18fefb80187787a1c0c8ed7ee3516744d24
Reviewed-on: https://skia-review.googlesource.com/57103
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
6 files changed