Allow Mojo structs as map keys

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

Index: mojo/public/tools/bindings/generators/cpp_templates/union_traits_declaration.tmpl
diff --git a/mojo/public/tools/bindings/generators/cpp_templates/union_traits_declaration.tmpl b/mojo/public/tools/bindings/generators/cpp_templates/union_traits_declaration.tmpl
index 583d5bebc1742d3f872336c77b3d57e6b1850547..ce54742003486256c104f4c89ff0c0f2a0e1a37a 100644
--- a/mojo/public/tools/bindings/generators/cpp_templates/union_traits_declaration.tmpl
+++ b/mojo/public/tools/bindings/generators/cpp_templates/union_traits_declaration.tmpl
@@ -14,10 +14,22 @@ struct {{export_attribute}} UnionTraits<{{mojom_type}}::DataView,
 {%-   set return_ref = field.kind|is_object_kind or
                        field.kind|is_any_handle_or_interface_kind %}
 {%-   if return_ref %}
+{%-     if field.kind|contains_handles %}
   static decltype(std::declval<{{mojom_type}}>().get_{{field.name}}())
       {{field.name}}({{mojom_type}}Ptr& input) {
     return input->get_{{field.name}}();
   }
+{%-     else %}
+{# We want the field accessor to be const whenever possible to allow
+   structs to be used as map keys. This complicated looking use of decltype
+   etc. gives us a const reference to the field type. #}
+{# TODO(tibell): Output the actual type of the field instead of using decltype
+   to compute it. #}
+  static std::add_lvalue_reference<const std::remove_reference<decltype(std::declval<{{mojom_type}}>().get_{{field.name}}())>::type>::type
+      {{field.name}}(const {{mojom_type}}Ptr& input) {
+    return input->get_{{field.name}}();
+  }
+{%-     endif %}
 {%-   else %}
   static decltype(std::declval<{{mojom_type}}>().get_{{field.name}}())
       {{field.name}}(const {{mojom_type}}Ptr& input) {