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

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

Index: mojo/public/cpp/bindings/lib/bindings_internal.h
diff --git a/mojo/public/cpp/bindings/lib/bindings_internal.h b/mojo/public/cpp/bindings/lib/bindings_internal.h
index 6f73ee6a16c180dd7aae7f418986816ce09c294b..2273d208f7c9e4beeab0ea696c750f713e34bf50 100644
--- a/mojo/public/cpp/bindings/lib/bindings_internal.h
+++ b/mojo/public/cpp/bindings/lib/bindings_internal.h
@@ -10,6 +10,7 @@
 #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"
@@ -330,6 +331,14 @@ struct EnumHashImpl {
   }
 };
 
+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