Add MGL entry points and port spinning_cube to use them

mojo/public/platform/native/BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 assert(!defined(is_nacl) || !is_nacl)
 
 import("../../mojo_sdk.gni")
 
 mojo_sdk_source_set("system") {
   sources = [
@@ skipping 87 matching lines @@
     "mojo/public/c/gles2:headers",
     "mojo/public/c/environment",
     "mojo/public/c/system",
   ]
 
   if (is_mac) {
     # TODO(GYP): Make it a run-path dependent library.
     # 'DYLIB_INSTALL_NAME_BASE': '@loader_path',
   }
 }
+
+mojo_sdk_source_set("gpu_thunks") {
+  sources = []
+}
+
+mojo_sdk_source_set("mgl_thunks") {
+  sources = [
+    "mgl_thunks.c",
+    "mgl_thunks.h",
+  ]
+
+  mojo_sdk_deps = [ "mojo/public/c/gpu:MGL" ]
+}
+
+mojo_sdk_source_set("mgl_onscreen_thunks") {
+  sources = [
+    "mgl_onscreen_thunks.c",
+    "mgl_onscreen_thunks.h",
+  ]
+
+  mojo_sdk_deps = [ "mojo/public/c/gpu:MGL_onscreen" ]
+}