[mojo] Allow value deserialization to fail

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

 size_t GetSerializedSize_(const {{union.name}}Ptr& input, bool inlined) {
   size_t size = 0U;
   if (!inlined) {
     size += sizeof(internal::{{union.name}}_Data);
   }
 
   if (!input)
     return size;
 
   mojo::internal::UnionAccessor<{{union.name}}> input_acc(input.get());
@@ skipping 75 matching lines @@
 {%- endfor %}
     }
   } else if (inlined) {
     result->set_null();
   } else {
     result = nullptr;
   }
   *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());
     switch (input->tag) {
 {%  for field in union.fields %}
       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}});
         result->set_{{field.name}}({{field.name}}->Pass());
 {%    elif field.kind|is_interface_kind %}
         {{field.kind|cpp_wrapper_type}} {{field.name}}_out;
         mojo::internal::Interface_Data* {{field.name}}_in =
             reinterpret_cast<mojo::internal::Interface_Data*>(
                 &input->data.f_{{field.name}});
         mojo::internal::InterfaceDataToPointer(
             {{field.name}}_in, &{{field.name}}_out);
         result->set_{{field.name}}({{field.name}}_out.Pass());
 {%-   elif field.kind|is_associated_kind %}
         // TODO(yzshen): add deserialization logic for associated kinds.
 {%    elif field.kind|is_enum_kind %}
         result->set_{{field.name}}(static_cast<{{field.kind|cpp_wrapper_type}}>(input->data.f_{{field.name}}));
 {%    else %}
         result->set_{{field.name}}(input->data.f_{{field.name}});
 {%-   endif %}
         break;
       }
 {%- endfor %}
     }
     *output = result.Pass();
   } else {
     output->reset();
   }
+  return success;
 }