implemented GrMtlBuffer for Metal gpu backend

Bug: skia:
Change-Id: Iff10fe69ccc08672cbbb06dad623a844008404cd
Reviewed-on: https://skia-review.googlesource.com/144606
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Timothy Liang <timliang@google.com>
diff --git a/gn/gpu.gni b/gn/gpu.gni
index 1c25078..fa00509 100644
--- a/gn/gpu.gni
+++ b/gn/gpu.gni
@@ -624,6 +624,8 @@
 
 skia_metal_sources = [
   "$_include/gpu/mtl/GrMtlTypes.h",
+  "$_src/gpu/mtl/GrMtlBuffer.h",
+  "$_src/gpu/mtl/GrMtlBuffer.mm",
   "$_src/gpu/mtl/GrMtlCaps.h",
   "$_src/gpu/mtl/GrMtlCaps.mm",
   "$_src/gpu/mtl/GrMtlCopyManager.h",