Add MGL entry points and port spinning_cube to use them

mojo/public/c/gpu/MGL/mgl_types.h

Index: mojo/public/c/gpu/MGL/mgl_types.h
diff --git a/mojo/public/c/gpu/MGL/mgl_types.h b/mojo/public/c/gpu/MGL/mgl_types.h
new file mode 100644
index 0000000000000000000000000000000000000000..22534564cd250f8a0830dcef8e5d6fd34e27fbae
--- /dev/null
+++ b/mojo/public/c/gpu/MGL/mgl_types.h
@@ -0,0 +1,22 @@
+// 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.
+
+// Note: This header should be compilable as C.
+
+#ifndef MOJO_PUBLIC_C_GPU_MGL_TYPES_H_
+#define MOJO_PUBLIC_C_GPU_MGL_TYPES_H_
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+typedef struct MGLContextPrivate* MGLContext;
+typedef void (*MGLContextLostCallback)(void* closure);
+typedef void (*MGLSignalSyncPointCallback)(void* closure);
+
+#ifdef __cplusplus
+}  // extern "C"
+#endif
+
+#endif  // MOJO_PUBLIC_C_GPU_MGL_TYPES_H_