Build sky/shell on Mac

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/1240823003 .
diff --git a/BUILD.gn b/BUILD.gn
index a99b27e..309428b 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -26,7 +26,7 @@
     deps += [ "//sky" ]
   }
 
-  if (is_ios) {
+  if (is_ios || is_mac) {
     deps = []
     deps = [
       "//sky/shell",