Allow Mojo structs as map keys

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

 {%- set mojom_type = struct|get_qualified_name_for_kind %}
 
 template <>
 struct {{export_attribute}} StructTraits<{{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(); }
 
 {%- for field in struct.fields %}
 {%-   set return_ref = field.kind|is_object_kind or
-                       field.kind|is_any_handle_or_interface_kind %}
+  field.kind|is_any_handle_or_interface_kind %}
+{#    We want the field accessor to be const whenever possible to allow
+      structs to be used as map keys.
+      TODO(tibell): Make this check more precise to deal with e.g.
+          custom types which don't contain handles but require non-const
+          reference for serialization. #}
+{%-   set maybe_const = "" if field.kind|contains_handles_or_interfaces else "const" %}
 {%-   if return_ref %}
-  static decltype({{mojom_type}}::{{field.name}})& {{field.name}}(
-      {{mojom_type}}Ptr& input) {
+  static {{maybe_const}} decltype({{mojom_type}}::{{field.name}})& {{field.name}}(
+      {{maybe_const}} {{mojom_type}}Ptr& input) {
     return input->{{field.name}};
   }
 {%-   else %}
   static decltype({{mojom_type}}::{{field.name}}) {{field.name}}(
       const {{mojom_type}}Ptr& input) {
     return input->{{field.name}};
   }
 {%-   endif %}
 {%- endfor %}
 
   static bool Read({{mojom_type}}::DataView input, {{mojom_type}}Ptr* output);
 };