Don't build sky_packager on Mac hosts

sky_packager requires C++11 and the Mac host toolchain can't handle C++11.

R=cstout@chromium.org, cstout@google.com

Review URL: https://codereview.chromium.org/1183223003.

BUILD.gn

@@ -14,12 +14,16 @@ group("sky") {
     "//sky/engine/web:sky_unittests",
     "//sky/engine/wtf:unittests",
     "//sky/tools/imagediff",
-    "//sky/tools/packager($host_toolchain)",
     "//sky/tools/tester",
     "//sky/viewer",
     ":sky_apk",
   ]
 
+  # Mac's host toolchain doesn't support C++11 and can't build the packager.
+  if (host_os != "mac") {
+    deps += [ "//sky/tools/packager($host_toolchain)" ]
+  }
+
   if (is_android) {
     deps += [ "//sky/apk/stocks" ]
   }