created stub for metal gpu command buffer

Bug: skia:
Change-Id: I11ffb1126ea4665c456c2b74e98d9e9021a5aca2
Reviewed-on: https://skia-review.googlesource.com/141080
Commit-Queue: Timothy Liang <timliang@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
diff --git a/gn/gpu.gni b/gn/gpu.gni
index b47fda9..6cac050 100644
--- a/gn/gpu.gni
+++ b/gn/gpu.gni
@@ -628,6 +628,7 @@
   "$_src/gpu/mtl/GrMtlCaps.mm",
   "$_src/gpu/mtl/GrMtlGpu.h",
   "$_src/gpu/mtl/GrMtlGpu.mm",
+  "$_src/gpu/mtl/GrMtlGpuCommandBuffer.h",
   "$_src/gpu/mtl/GrMtlRenderTarget.mm",
   "$_src/gpu/mtl/GrMtlRenderTarget.h",
   "$_src/gpu/mtl/GrMtlTexture.mm",