Unify ShadowUtils interface

Bug: skia:
Change-Id: I116bec82783d297e91ef061217b5e61f7ff16a76
Reviewed-on: https://skia-review.googlesource.com/16371
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
5 files changed