SkColorSpaceXformSteps improvements and tests

Added some helper functions and put the booleans in a nested struct
(this is motivated by upcoming changes). Added a unit test of steps
against skcms, with round-tripping in both combinations.

Change-Id: Iea3d60cd52edb5259b5576b1422ed6f856cde815
Reviewed-on: https://skia-review.googlesource.com/134660
Reviewed-by: Mike Klein <mtklein@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
diff --git a/gn/tests.gni b/gn/tests.gni
index 409d231..5f028e8 100644
--- a/gn/tests.gni
+++ b/gn/tests.gni
@@ -139,6 +139,7 @@
   "$_tests/MessageBusTest.cpp",
   "$_tests/MetaDataTest.cpp",
   "$_tests/MipMapTest.cpp",
+  "$_tests/NonlinearBlendingTest.cpp",
   "$_tests/OnceTest.cpp",
   "$_tests/OSPathTest.cpp",
   "$_tests/OverAlignedTest.cpp",