implemented GrMtlGpuCommandBuffer for draws in Metal gpu backend

Bug: skia:
Change-Id: Ifc20a7734a02fe48ef79af201b4ebc882aedefb3
Reviewed-on: https://skia-review.googlesource.com/146382
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 e6533ec..fc115fb 100644
--- a/gn/gpu.gni
+++ b/gn/gpu.gni
@@ -634,6 +634,7 @@
   "$_src/gpu/mtl/GrMtlGpu.h",
   "$_src/gpu/mtl/GrMtlGpu.mm",
   "$_src/gpu/mtl/GrMtlGpuCommandBuffer.h",
+  "$_src/gpu/mtl/GrMtlGpuCommandBuffer.mm",
   "$_src/gpu/mtl/GrMtlPipelineState.h",
   "$_src/gpu/mtl/GrMtlPipelineState.mm",
   "$_src/gpu/mtl/GrMtlPipelineStateBuilder.h",