Use SkToInt to avoid warning in Flutter roll.

The Flutter roll was failing due to -Wsign-compare.

Bug: chromium:1505053
Change-Id: Id12876f6f97682466f19b56cfa562366380f27cb
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/783036
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>