rewrote GrAlphaThresholdFragmentProcessor in sksl

Bug: skia:
Change-Id: I641b206fc3bc19ac190ad94ee755ab9e1caab9b3
Reviewed-on: https://skia-review.googlesource.com/21341
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
diff --git a/gn/sksl.gni b/gn/sksl.gni
index 8c74b30..6aede6e 100644
--- a/gn/sksl.gni
+++ b/gn/sksl.gni
@@ -23,4 +23,7 @@
   "$_src/sksl/ir/SkSLType.cpp",
 ]
 
-skia_gpu_processor_sources = [ "$_src/gpu/effects/GrDitherEffect.fp" ]
+skia_gpu_processor_sources = [
+  "$_src/effects/GrAlphaThresholdFragmentProcessor.fp",
+  "$_src/gpu/effects/GrDitherEffect.fp",
+]