SkSL FPs now support child processors, converted ArithmeticFP to SkSL

Bug: skia:
Change-Id: I34ed3480073d05762a7d4692aeee4b87e454ce52
Reviewed-on: https://skia-review.googlesource.com/57961
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
14 files changed