converted GrMagnifierEffect to SkSL

Bug: skia:
Change-Id: I6dc14ac66d5b911117e71fa23fef49a897082781
Reviewed-on: https://skia-review.googlesource.com/71342
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
diff --git a/gn/sksl.gni b/gn/sksl.gni
index e40da5d..9d992d1 100644
--- a/gn/sksl.gni
+++ b/gn/sksl.gni
@@ -37,6 +37,7 @@
   "$_src/gpu/effects/GrDitherEffect.fp",
   "$_src/gpu/effects/GrEllipseEffect.fp",
   "$_src/gpu/effects/GrLumaColorFilterEffect.fp",
+  "$_src/gpu/effects/GrMagnifierEffect.fp",
   "$_src/gpu/effects/GrPremulInputFragmentProcessor.fp",
   "$_src/gpu/effects/GrRectBlurEffect.fp",
   "$_src/gpu/effects/GrOverdrawFragmentProcessor.fp",