Add MGL entry points and port spinning_cube to use them

mojo/public/c/gpu/GLES2/gl2extmojo.h

Index: mojo/public/c/gpu/GLES2/gl2extmojo.h
diff --git a/gpu/GLES2/gl2extchromium.h b/mojo/public/c/gpu/GLES2/gl2extmojo.h
similarity index 99%
copy from gpu/GLES2/gl2extchromium.h
copy to mojo/public/c/gpu/GLES2/gl2extmojo.h
index ec85bf2d4b0a755febc5df7f9e3fcf91e63f0fa8..be0533e8bf9d959803122faac9ef2ecd31774d4b 100644
--- a/gpu/GLES2/gl2extchromium.h
+++ b/mojo/public/c/gpu/GLES2/gl2extmojo.h
@@ -2,10 +2,10 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// This file contains Chromium-specific GLES2 extensions declarations.
+// This file contains Mojo-specific GLES2 extensions declarations.
 
-#ifndef GPU_GLES2_GL2EXTCHROMIUM_H_
-#define GPU_GLES2_GL2EXTCHROMIUM_H_
+#ifndef GPU_GLES2_GL2EXTMOJO_H_
+#define GPU_GLES2_GL2EXTMOJO_H_
 
 #include <GLES2/gl2.h>
 #include <GLES2/gl2ext.h>
@@ -785,4 +785,4 @@ typedef void(GL_APIENTRYP PFNGLMATRIXLOADIDENTITYCHROMIUMPROC)(
 }
 #endif
 
-#endif  // GPU_GLES2_GL2EXTCHROMIUM_H_
+#endif  // GPU_GLES2_GL2EXTMOJO_H_