[mojo] Allow value deserialization to fail

mojo/public/tools/bindings/generators/cpp_templates/union_serialization_definition.tmpl

Index: mojo/public/tools/bindings/generators/cpp_templates/union_serialization_definition.tmpl
diff --git a/mojo/public/tools/bindings/generators/cpp_templates/union_serialization_definition.tmpl b/mojo/public/tools/bindings/generators/cpp_templates/union_serialization_definition.tmpl
index 3471932b6818d57bc872b80797486be1e731527d..73f6c95865fb763d5c2f25440c85fbbae7fb448b 100644
--- a/mojo/public/tools/bindings/generators/cpp_templates/union_serialization_definition.tmpl
+++ b/mojo/public/tools/bindings/generators/cpp_templates/union_serialization_definition.tmpl
@@ -93,9 +93,10 @@ void SerializeUnion_({{union.name}}Ptr input, mojo::internal::Buffer* buf,
   *output = result;
 }
 
-void Deserialize_(internal::{{union.name}}_Data* input,
+bool Deserialize_(internal::{{union.name}}_Data* input,
                   {{union.name}}Ptr* output,
                   mojo::internal::SerializationContext* context) {
+  bool success = true;
   if (input && !input->is_null()) {
     {{union.name}}Ptr result({{union.name}}::New());
     mojo::internal::UnionAccessor<{{union.name}}> result_acc(result.get());
@@ -104,7 +105,8 @@ void Deserialize_(internal::{{union.name}}_Data* input,
       case {{union.name}}::Tag::{{field.name|upper}}: {
 {%    if field.kind|is_object_kind %}
         result_acc.SwitchActive({{union.name}}::Tag::{{field.name|upper}});
-        Deserialize_(input->data.f_{{field.name}}.ptr, result_acc.data()->{{field.name}}, context);
+        if (!Deserialize_(input->data.f_{{field.name}}.ptr, result_acc.data()->{{field.name}}, context))
+          success = false;
 {%    elif field.kind|is_any_handle_kind %}
         {{field.kind|cpp_wrapper_type}}* {{field.name}} =
             reinterpret_cast<{{field.kind|cpp_wrapper_type}}*>(&input->data.f_{{field.name}});
@@ -132,4 +134,5 @@ void Deserialize_(internal::{{union.name}}_Data* input,
   } else {
     output->reset();
   }
+  return success;
 }