Work around signed-unsigned comparison warning.

Change-Id: I42f21544f6b626d3b75b65bcccde0b5066dc520f
Reviewed-on: https://skia-review.googlesource.com/135320
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
1 file changed