Mojo C++ bindings: support disconnect with a reason.

mojo/public/cpp/bindings/connection_error_callback.h

+// Copyright 2016 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_CONNECTION_ERROR_CALLBACK_H_
+#define MOJO_PUBLIC_CPP_BINDINGS_CONNECTION_ERROR_CALLBACK_H_
+
+#include "base/callback.h"
+
+namespace mojo {
+
+// This callback type accepts user-defined disconnect reason and description. If
+// the other side specifies a reason on closing the connection, it will be
+// passed to the error handler.
+using ConnectionErrorWithReasonCallback =
+    base::Callback<void(uint32_t /* custom_reason */,
+                        const std::string& /* description */)>;
+
+}  // namespace mojo
+
+#endif  // MOJO_PUBLIC_CPP_BINDINGS_CONNECTION_ERROR_CALLBACK_H_