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 %}
-  static decltype(std::declval<{{mojom_type}}>().get_{{field.name}}())
-      {{field.name}}({{mojom_type}}Ptr& input) {
+{%-   set maybe_const_in = "" if field.kind|contains_handles else "const" %}
+{%-   set maybe_const_out = "" if field.kind|contains_handles or not field.kind|is_reference_kind else "const" %}
+{# We want the field accessor to be const whenever possible to allow
+   structs to be used as map keys. #}
+  static {{maybe_const_out}} {{field.kind|cpp_union_trait_getter_return_type}} {{field.name}}({{maybe_const_in}} {{mojom_type}}Ptr& input) {
     return input->get_{{field.name}}();
   }
-{%-   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);
 };