add helper makeShader for the common Clamp case

Bug: skia:
Change-Id: I22cac56f35fed16daede0cf1b3e34dfa6f5ae3ea
Reviewed-on: https://skia-review.googlesource.com/14625
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
12 files changed