Add GrMtlUtil class

Currently just adding support functions to go back and forth between
GrPixelConfigs and MTLPixelFormats.

Bug: skia:
Change-Id: I01a7d6877ebed87b87090ac2b920fee45dc0e856
Reviewed-on: https://skia-review.googlesource.com/29080
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
diff --git a/gn/gpu.gni b/gn/gpu.gni
index 49e3361..9bd28e1 100644
--- a/gn/gpu.gni
+++ b/gn/gpu.gni
@@ -587,6 +587,8 @@
   "$_src/gpu/mtl/GrMtlGpu.mm",
   "$_src/gpu/mtl/GrMtlTrampoline.h",
   "$_src/gpu/mtl/GrMtlTrampoline.mm",
+  "$_src/gpu/mtl/GrMtlUtil.h",
+  "$_src/gpu/mtl/GrMtlUtil.mm",
 ]
 
 skia_native_gpu_sources = [