Add basic metal build files for backend

Bug: skia:
Change-Id: Iddeeb91b378bdb61d200070d8faa3610299ab733
Reviewed-on: https://skia-review.googlesource.com/21533
Reviewed-by: Brian Osman <brianosman@google.com>
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 e4ffe2f..622d8d1 100644
--- a/gn/gpu.gni
+++ b/gn/gpu.gni
@@ -559,6 +559,13 @@
   "$_src/gpu/vk/GrVkVertexBuffer.h",
 ]
 
+skia_metal_sources = [
+  "$_src/gpu/mtl/GrMtlGpu.h",
+  "$_src/gpu/mtl/GrMtlGpu.mm",
+  "$_src/gpu/mtl/GrMtlTrampoline.h",
+  "$_src/gpu/mtl/GrMtlTrampoline.mm",
+]
+
 skia_native_gpu_sources = [
   "$_src/gpu/gl/GrGLDefaultInterface_native.cpp",
   "$_src/gpu/gl/mac/GrGLCreateNativeInterface_mac.cpp",