use unsigned to avoid runtime overflow detection

Bug: skia:
Change-Id: I487930955f75048ea27a1bcc61f7e0849c63759b
Reviewed-on: https://skia-review.googlesource.com/32681
Commit-Queue: Mike Reed <reed@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
1 file changed