converted CircleEffect to SkSL

Bug: skia:
Change-Id: I93d117c22ae2b374294f6a5e961c497ac2c92b09
Reviewed-on: https://skia-review.googlesource.com/23301
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
6 files changed