Support exporting Mojo bindings for the component build.

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

 {%- import "struct_macros.tmpl" as struct_macros %}
 {%- set mojom_type = struct|get_qualified_name_for_kind %}
 {%- set data_type = struct|get_qualified_name_for_kind(internal=True) %}
 
 template <>
-struct StructTraits<{{mojom_type}}, {{mojom_type}}Ptr> {
+struct {{export_attribute}} StructTraits<{{mojom_type}}, {{mojom_type}}Ptr> {
   static bool IsNull(const {{mojom_type}}Ptr& input) { return !input; }
   static void SetToNull({{mojom_type}}Ptr* output) { output->reset(); }
 
 {%- for field in struct.fields %}
 {%-   set return_ref = field.kind|is_object_kind or
                        field.kind|is_any_handle_or_interface_kind %}
 {%-   if return_ref %}
   static decltype({{mojom_type}}::{{field.name}})& {{field.name}}(
       {{mojom_type}}Ptr& input) {
     return input->{{field.name}};
@@ skipping 55 matching lines @@
                           SerializationContext* context) {
     if (!input)
       return CallSetToNullIfExists<Traits>(output);
 
     {{mojom_type}}DataView data_view(input, context);
     return Traits::Read(data_view, output);
   }
 };
 
 }  // namespace internal