Add gm to compare premuls in dst vs. src color space

BUG=skia:

Change-Id: I36755f4bbc405a8af12990573e1f554df012b30a
Reviewed-on: https://skia-review.googlesource.com/9402
Commit-Queue: Matt Sarett <msarett@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
diff --git a/gn/gm.gni b/gn/gm.gni
index 32c22d7..3b94be5 100644
--- a/gn/gm.gni
+++ b/gn/gm.gni
@@ -129,6 +129,7 @@
   "$_gm/fontscaler.cpp",
   "$_gm/fontscalerdistortable.cpp",
   "$_gm/gamma.cpp",
+  "$_gm/gammaencodedpremul.cpp",
   "$_gm/gammatext.cpp",
   "$_gm/gamut.cpp",
   "$_gm/gaussianedge.cpp",