Revert of Deletes mojo::Callback

mojo/public/cpp/bindings/callback.h

Index: mojo/public/cpp/bindings/callback.h
diff --git a/mojo/public/cpp/bindings/callback.h b/mojo/public/cpp/bindings/callback.h
new file mode 100644
index 0000000000000000000000000000000000000000..f918d7d540f68cef679ab62c3408ec30edf3c7aa
--- /dev/null
+++ b/mojo/public/cpp/bindings/callback.h
@@ -0,0 +1,19 @@
+// 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.
+
+#ifndef MOJO_PUBLIC_CPP_BINDINGS_CALLBACK_H_
+#define MOJO_PUBLIC_CPP_BINDINGS_CALLBACK_H_
+
+#include "base/callback.h"
+
+namespace mojo {
+
+template <typename Sig>
+using Callback = base::Callback<Sig>;
+
+using Closure = base::Closure;
+
+}  // namespace mojo
+
+#endif  // MOJO_PUBLIC_CPP_BINDINGS_CALLBACK_H_