Add Alpha Image GM

Change-Id: I835a59d9f8593e30db817dc8bd1d276af8c80cd7
Reviewed-on: https://skia-review.googlesource.com/24685
Reviewed-by: Hal Canary <halcanary@google.com>
Commit-Queue: Hal Canary <halcanary@google.com>
diff --git a/gn/gm.gni b/gn/gm.gni
index a667b89..7fd787c 100644
--- a/gn/gm.gni
+++ b/gn/gm.gni
@@ -13,6 +13,7 @@
   "$_gm/aaxfermodes.cpp",
   "$_gm/addarc.cpp",
   "$_gm/all_bitmap_configs.cpp",
+  "$_gm/alpha_image.cpp",
   "$_gm/alphagradients.cpp",
   "$_gm/animatedGif.cpp",
   "$_gm/animatedimageblurs.cpp",