Add MGL entry points and port spinning_cube to use them

mojo/public/platform/native/mgl_thunks.c

Index: mojo/public/platform/native/mgl_thunks.c
diff --git a/mojo/public/platform/native/mgl_thunks.c b/mojo/public/platform/native/mgl_thunks.c
new file mode 100644
index 0000000000000000000000000000000000000000..2e8e9d84c37e3aa3c97f73d03f7616ffccfb87fe
--- /dev/null
+++ b/mojo/public/platform/native/mgl_thunks.c
@@ -0,0 +1,45 @@
+// Copyright 2015 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.
+
+#include "mojo/public/platform/native/mgl_thunks.h"
+
+#include <assert.h>
+
+#include "mojo/public/platform/native/thunk_export.h"
+
+static struct MGLThunks g_thunks = {0};
+
+MGLContext MGLCreateContext(MGLOpenGLAPIVersion version,
+                            MojoHandle command_buffer_handle,
+                            MGLContext share_group,
+                            MGLContextLostCallback lost_callback,
+                            void* lost_callback_closure,
+                            const struct MojoAsyncWaiter* async_waiter) {
+  assert(g_thunks.MGLCreateContext);
+  return g_thunks.MGLCreateContext(
+      version, command_buffer_handle, share_group, lost_callback,
+      lost_callback_closure, async_waiter);
+}
+
+void MGLDestroyContext(MGLContext context) {
+  assert(g_thunks.MGLDestroyContext);
+  g_thunks.MGLDestroyContext(context);
+}
+
+void MGLMakeCurrent(MGLContext context) {
+  assert(g_thunks.MGLMakeCurrent);
+  g_thunks.MGLMakeCurrent(context);
+}
+
+MGLContext MGLGetCurrentContext(void) {
+  assert(g_thunks.MGLGetCurrentContext);
+  return g_thunks.MGLGetCurrentContext();
+}
+
+THUNK_EXPORT size_t MojoSetMGLThunks(
+    const struct MGLThunks* mgl_thunks) {
+  if (mgl_thunks->size >= sizeof(g_thunks))
+    g_thunks = *mgl_thunks;
+  return sizeof(g_thunks);
+}