Add GN config for Mac SampleApp

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4077

Change-Id: I67e4456c8ccdb5413e86c442b759f6dff4de651b
Reviewed-on: https://skia-review.googlesource.com/4077
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Cary Clark <caryclark@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index 7f0d16e..e57cd0e 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -992,7 +992,7 @@
     testonly = true
   }
 
-  if (is_linux || is_win) {
+  if (is_linux || is_win || is_mac) {
     executable("SampleApp") {
       sources = [
         "samplecode/SampleApp.cpp",