Allow Mojo structs as map keys

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

 {%- set mojom_type = union|get_qualified_name_for_kind %}
 
 template <>
 struct {{export_attribute}} UnionTraits<{{mojom_type}}::DataView,
                                         {{mojom_type}}Ptr> {
   static bool IsNull(const {{mojom_type}}Ptr& input) { return !input; }
   static void SetToNull({{mojom_type}}Ptr* output) { output->reset(); }
 
   static {{mojom_type}}::Tag GetTag(const {{mojom_type}}Ptr& input) {
     return input->which();
   }
 
 {%- for field in union.fields %}
 {%-   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) {
     return input->get_{{field.name}}();
   }
 {%-   endif %}
 {%- endfor %}
 
   static bool Read({{mojom_type}}::DataView input, {{mojom_type}}Ptr* output);
 };