Fix signed/unsigned comparison in assert

Bug: skia:
Change-Id: I4759ea1c42cad1d1b1f159d1d8c2c607453abad7
Reviewed-on: https://skia-review.googlesource.com/66900
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
1 file changed