Wire up SDL on mac

third_party/libsdl/sdl.gypi

 # Copyright 2015 Google Inc.
 #
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 {
   'targets': [
     {
       'target_name': 'sdl',
       'product_name': 'sdl',
@@ skipping 16 matching lines @@
         ['skia_os == "linux"', {
           'includes': [
             'linux/sdl_linux.gypi',
           ]
         }],
         ['skia_os == "android"', {
           'includes': [
             'android/sdl_android.gypi',
           ]
         }],
+        ['skia_os == "mac"', {
+          'includes': [
+            'mac/sdl_mac.gypi',
+          ]
+        }],
       ],
     },
   ],
 }