Mojo C++ bindings: add support for UnionTraits.

mojo/public/cpp/bindings/union_traits.h

Index: mojo/public/cpp/bindings/union_traits.h
diff --git a/mojo/public/cpp/bindings/union_traits.h b/mojo/public/cpp/bindings/union_traits.h
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+++ b/mojo/public/cpp/bindings/union_traits.h
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+// 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_UNION_TRAITS_H_
+#define MOJO_PUBLIC_CPP_BINDINGS_UNION_TRAITS_H_
+
+namespace mojo {
+
+// This must be specialized for any type |T| to be serialized/deserialized as
+// a mojom union of type |MojomType|.
+//
+// Similar to StructTraits, each specialization of UnionTraits implements the
+// following methods:
+//   1. Getters for each field in the Mojom type.
+//   2. Read() method.
+//   3. [Optional] IsNull() and SetToNull().
+//   4. [Optional] SetUpContext() and TearDownContext().
+// Please see the documentation of StructTraits for details of these methods.
+//
+// Unlike StructTraits, there is one more method to implement:
+//   5. A static GetTag() method indicating which field is the current active
+//      field for serialization:
+//
+//        static |MojomType|DataView::Tag GetTag(const T& input);
+//
+//      During serialization, only the field getter corresponding to this tag
+//      will be called.
+//
+template <typename MojomType, typename T>
+struct UnionTraits;
+
+}  // namespace mojo
+
+#endif  // MOJO_PUBLIC_CPP_BINDINGS_UNION_TRAITS_H_