Implement sweep gradient layout

Add an FP that implements the sweep gradient effect and updates
SkSweepGradient to use the new system if possible.

Bug: skia:
Change-Id: I3f65da01afafae54c45848a6a78fd758f65eb4a6
Reviewed-on: https://skia-review.googlesource.com/148806
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
diff --git a/gn/gpu.gni b/gn/gpu.gni
index 463b5ed..f2005e5 100644
--- a/gn/gpu.gni
+++ b/gn/gpu.gni
@@ -407,6 +407,8 @@
   "$_src/gpu/gradients/GrLinearGradientLayout.h",
   "$_src/gpu/gradients/GrRadialGradientLayout.cpp",
   "$_src/gpu/gradients/GrRadialGradientLayout.h",
+  "$_src/gpu/gradients/GrSweepGradientLayout.cpp",
+  "$_src/gpu/gradients/GrSweepGradientLayout.h",
   "$_src/gpu/gradients/GrClampedGradientEffect.cpp",
   "$_src/gpu/gradients/GrClampedGradientEffect.h",
   "$_src/gpu/gradients/GrTiledGradientEffect.cpp",