Use a different GN version on macOS

GN version 507254 doesn't seem to be published in GN.

Change-Id: I6848d06eebbaf8659227e3e33ddd803577645fb8
diff --git a/fuchsia.ensure b/fuchsia.ensure
index b26f4b8..434aaa4 100644
--- a/fuchsia.ensure
+++ b/fuchsia.ensure
@@ -12,7 +12,8 @@
 infra/ninja/${platform} version:1.8.2
 
 # GN
-infra/tools/gn/${platform} gn_version:507254
+infra/tools/gn/${os=linux}-${arch} gn_version:507254
+infra/tools/gn/${os=mac}-${arch} gn_version:499403
 
 # godepfile
 fuchsia/tools/godepfile/${platform} git_revision:6922d7833617841e853a0be52a285f6cd07a0a10