Add GM to test ETC1

I don't think ETC1 currently gets tested any other way.

Change-Id: I125d6cb52603c87fc91b47d02743dc4efe0409b7
Reviewed-on: https://skia-review.googlesource.com/8316
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
diff --git a/gn/gm.gni b/gn/gm.gni
index b1d3c4ad..09a7963 100644
--- a/gn/gm.gni
+++ b/gn/gm.gni
@@ -114,6 +114,7 @@
   "$_gm/encode.cpp",
   "$_gm/encode-platform.cpp",
   "$_gm/encode-srgb.cpp",
+  "$_gm/etc1.cpp",
   "$_gm/extractbitmap.cpp",
   "$_gm/fadefilter.cpp",
   "$_gm/fatpathfill.cpp",