Add animated cowboy sample from WebKit tests, and fix.

Bug: chromium:712455
Change-Id: Ic9bb9b862abe01f112cc41d28589733460b15bc1
Reviewed-on: https://skia-review.googlesource.com/50181
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
diff --git a/gn/samples.gni b/gn/samples.gni
index b61ec34..6e1bcac 100644
--- a/gn/samples.gni
+++ b/gn/samples.gni
@@ -38,6 +38,7 @@
   "$_samplecode/SampleColorFilter.cpp",
   "$_samplecode/SampleComplexClip.cpp",
   "$_samplecode/SampleConcavePaths.cpp",
+  "$_samplecode/SampleCowboy.cpp",
   "$_samplecode/SampleDegenerateTwoPtRadials.cpp",
   "$_samplecode/SampleDither.cpp",
   "$_samplecode/SampleEffects.cpp",