Mojo C++ Bindings: Support custom impl ref types

mojo/public/cpp/bindings/raw_ptr_impl_ref_traits.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_RAW_PTR_IMPL_REF_TRAITS_H_
+#define MOJO_PUBLIC_CPP_BINDINGS_RAW_PTR_IMPL_REF_TRAITS_H_
+
+namespace mojo {
+
+// Default traits for a binding's implementation reference type. This
+// corresponds to a raw pointer.
+template <typename Interface>
+struct RawPtrImplRefTraits {
+  using PointerType = Interface*;
+
+  static bool IsNull(PointerType ptr) { return !ptr; }
+  static Interface* GetRawPointer(PointerType* ptr) { return *ptr; }
+};
+
+}  // namespace mojo
+
+#endif  // MOJO_PUBLIC_CPP_BINDINGS_LIB_RAW_PTR_IMPL_REF_TRAITS_H_