added TextureRenderTarget to Metal gpu backend

Bug: skia:
Change-Id: I5cacdb832deefe0f8d8460ff10b2216d5dec1ed7
Reviewed-on: https://skia-review.googlesource.com/137890
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 7e5dd62..76b9a16 100644
--- a/gn/gpu.gni
+++ b/gn/gpu.gni
@@ -635,6 +635,8 @@
   "$_src/gpu/mtl/GrMtlRenderTarget.h",
   "$_src/gpu/mtl/GrMtlTexture.mm",
   "$_src/gpu/mtl/GrMtlTexture.h",
+  "$_src/gpu/mtl/GrMtlTextureRenderTarget.mm",
+  "$_src/gpu/mtl/GrMtlTextureRenderTarget.h",
   "$_src/gpu/mtl/GrMtlTrampoline.h",
   "$_src/gpu/mtl/GrMtlTrampoline.mm",
   "$_src/gpu/mtl/GrMtlUtil.h",