add Start/EndFragmentProcessor to DSLWriter

Change-Id: I6f9d414d55b3ba64c3f67f3a590f6b7b5642062a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/364599
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
diff --git a/gn/sksl.gni b/gn/sksl.gni
index 0cfde02..1c2e74f 100644
--- a/gn/sksl.gni
+++ b/gn/sksl.gni
@@ -74,6 +74,8 @@
   "$_src/sksl/dsl/DSLType.h",
   "$_src/sksl/dsl/DSLVar.cpp",
   "$_src/sksl/dsl/DSLVar.h",
+  "$_src/sksl/dsl/priv/DSLFPs.cpp",
+  "$_src/sksl/dsl/priv/DSLFPs.h",
   "$_src/sksl/dsl/priv/DSLWriter.cpp",
   "$_src/sksl/dsl/priv/DSLWriter.h",
   "$_src/sksl/ir/SkSLBinaryExpression.h",