Use enum value as the lower bound

This is triggering a compilation error in the latest version of Clang
because of the new Wtautological-unsigned-zero-compare warning.

Change-Id: I578eb1244719fd331113b9444e2eca672912afc8
Reviewed-on: https://skia-review.googlesource.com/44580
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
1 file changed