Mojo C++ bindings: support setting default value for fields whose types are typemapped enums.

mojo/public/cpp/bindings/lib/bindings_internal.h

 // Copyright 2013 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_LIB_BINDINGS_INTERNAL_H_
 #define MOJO_PUBLIC_CPP_BINDINGS_LIB_BINDINGS_INTERNAL_H_
 
 #include <stdint.h>
 
 #include <functional>
 #include <type_traits>
 
+#include "mojo/public/cpp/bindings/enum_traits.h"
 #include "mojo/public/cpp/bindings/interface_id.h"
 #include "mojo/public/cpp/bindings/lib/template_util.h"
 #include "mojo/public/cpp/system/core.h"
 
 namespace mojo {
 
 template <typename T>
 class ArrayDataView;
 
 template <typename T>
@@ skipping 300 matching lines @@
 template <typename T>
 struct EnumHashImpl {
   static_assert(std::is_enum<T>::value, "Incorrect hash function.");
 
   size_t operator()(T input) const {
     using UnderlyingType = typename std::underlying_type<T>::type;
     return std::hash<UnderlyingType>()(static_cast<UnderlyingType>(input));
   }
 };
 
+template <typename MojomType, typename T>
+T ConvertEnumValue(MojomType input) {
+  T output;
+  bool result = EnumTraits<MojomType, T>::FromMojom(input, &output);
+  CHECK(result);

[Ken Rockot(use gerrit already), 2017/06/29 22:49:39]

nit: I think we should limit this to DCHECK given that I don't think it's a
critical security bug for someone to initialize an enum with a bad value.

[yzshen1, 2017/06/29 22:51:39]

Done.

+  return output;
+}
+
 }  // namespace internal
 }  // namespace mojo
 
 #endif  // MOJO_PUBLIC_CPP_BINDINGS_LIB_BINDINGS_INTERNAL_H_