[mojo] Allow value deserialization to fail

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

 {%- import "struct_macros.tmpl" as struct_macros %}
 size_t GetSerializedSize_(const {{struct.name}}Ptr& input) {
   if (!input)
     return 0;
   {{struct_macros.get_serialized_size(struct, "input->%s")}}
   return size;
 }
 
 void Serialize_({{struct.name}}Ptr input, mojo::internal::Buffer* buf,
                 internal::{{struct.name}}_Data** output) {
   if (input) {
     {{struct_macros.serialize(struct, struct.name ~ " struct", "input->%s", "result", "buf")|indent(2)}}
     *output = result;
   } else {
     *output = nullptr;
   }
 }
 
-void Deserialize_(internal::{{struct.name}}_Data* input,
+bool Deserialize_(internal::{{struct.name}}_Data* input,
                   {{struct.name}}Ptr* output,
                   mojo::internal::SerializationContext* context) {
+  bool success = true;
   if (input) {
     {{struct.name}}Ptr result({{struct.name}}::New());
-    {{struct_macros.deserialize(struct, "input", "result->%s", "context")|indent(2)}}
+    {{struct_macros.deserialize(struct, "input", "result->%s", "context", "success")|indent(2)}}
     *output = result.Pass();
   } else {
     output->reset();
   }
+  return success;
 }