preliminary SkSL Metal backend

Bug: skia:
Change-Id: I03e839fb4f1061bc6d1b1b72d54fd39ca7bd7b84
Reviewed-on: https://skia-review.googlesource.com/51245
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
diff --git a/gn/sksl.gni b/gn/sksl.gni
index 615b5dc..a508783 100644
--- a/gn/sksl.gni
+++ b/gn/sksl.gni
@@ -15,6 +15,7 @@
   "$_src/sksl/SkSLIRGenerator.cpp",
   "$_src/sksl/SkSLLexer.cpp",
   "$_src/sksl/SkSLLayoutLexer.cpp",
+  "$_src/sksl/SkSLMetalCodeGenerator.cpp",
   "$_src/sksl/SkSLParser.cpp",
   "$_src/sksl/SkSLSPIRVCodeGenerator.cpp",
   "$_src/sksl/SkSLString.cpp",