implemented onWritePixels and related tests for metal gpu backend

Bug: skia:
Change-Id: Iab8bc8611a9de8afcf0eaeb58a8ef46fdc38d4f1
Reviewed-on: https://skia-review.googlesource.com/140571
Commit-Queue: Timothy Liang <timliang@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
diff --git a/gn/tests.gni b/gn/tests.gni
index 6b08b82..b78cc57 100644
--- a/gn/tests.gni
+++ b/gn/tests.gni
@@ -105,6 +105,7 @@
   "$_tests/GrTestingBackendTextureUploadTest.cpp",
   "$_tests/GrTextureMipMapInvalidationTest.cpp",
   "$_tests/GrTRecorderTest.cpp",
+  "$_tests/GrUploadPixelsTests.cpp",
   "$_tests/HashTest.cpp",
   "$_tests/HighContrastFilterTest.cpp",
   "$_tests/HSVRoundTripTest.cpp",
@@ -277,7 +278,6 @@
   "$_tests/VerticesTest.cpp",
   "$_tests/VkBackendSurfaceTest.cpp",
   "$_tests/VkMakeCopyPipelineTest.cpp",
-  "$_tests/VkUploadPixelsTests.cpp",
   "$_tests/VkWrapTests.cpp",
   "$_tests/VptrTest.cpp",
   "$_tests/WindowRectanglesTest.cpp",