Re-re-land "added GrSkSLFP and converted DitherEffect to use it"

This reverts commit 6c48e4d11ce80fa5cfef56e13b2d5847fe94a7cc.

Bug: skia:
Change-Id: I7ee78990fc30eec545d1856e59eb6e0573089426
Reviewed-on: https://skia-review.googlesource.com/144348
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
diff --git a/gn/gpu.gni b/gn/gpu.gni
index 6cac050..51a0dc2 100644
--- a/gn/gpu.gni
+++ b/gn/gpu.gni
@@ -364,8 +364,6 @@
   "$_src/gpu/effects/GrDisableColorXP.h",
   "$_src/gpu/effects/GrDistanceFieldGeoProc.cpp",
   "$_src/gpu/effects/GrDistanceFieldGeoProc.h",
-  "$_src/gpu/effects/GrDitherEffect.cpp",
-  "$_src/gpu/effects/GrDitherEffect.h",
   "$_src/gpu/effects/GrEllipseEffect.cpp",
   "$_src/gpu/effects/GrEllipseEffect.h",
   "$_src/gpu/effects/GrGaussianConvolutionFragmentProcessor.cpp",
@@ -394,6 +392,8 @@
   "$_src/gpu/effects/GrShadowGeoProc.h",
   "$_src/gpu/effects/GrSimpleTextureEffect.cpp",
   "$_src/gpu/effects/GrSimpleTextureEffect.h",
+  "$_src/gpu/effects/GrSkSLFP.cpp",
+  "$_src/gpu/effects/GrSkSLFP.h",
   "$_src/gpu/effects/GrSRGBEffect.cpp",
   "$_src/gpu/effects/GrSRGBEffect.h",
   "$_src/gpu/effects/GrTextureDomain.cpp",