converted GrCircleBlurFragmentProcessor to sksl

Bug: skia:
Change-Id: I1b70ba2003c9e9de2b5b9acadaf25c9ed59b1198
Reviewed-on: https://skia-review.googlesource.com/21727
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
12 files changed