Revert "Revert "Clean up onTransferPixels""

Bug: skia:5126
Change-Id: Ia1eaef56cca266ad4c413e711e63646e913222be
Reviewed-on: https://skia-review.googlesource.com/20445
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
diff --git a/gn/tests.gni b/gn/tests.gni
index 80af714..6843a6f 100644
--- a/gn/tests.gni
+++ b/gn/tests.gni
@@ -252,6 +252,7 @@
   "$_tests/TopoSortTest.cpp",
   "$_tests/TraceMemoryDumpTest.cpp",
   "$_tests/TracingTest.cpp",
+  "$_tests/TransferPixelsTest.cpp",
   "$_tests/TypefaceTest.cpp",
   "$_tests/UnicodeTest.cpp",
   "$_tests/UtilsTest.cpp",