Allow Mojo structs as map keys

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

Index: mojo/public/tools/bindings/generators/cpp_templates/struct_traits_declaration.tmpl
diff --git a/mojo/public/tools/bindings/generators/cpp_templates/struct_traits_declaration.tmpl b/mojo/public/tools/bindings/generators/cpp_templates/struct_traits_declaration.tmpl
index 24efbd5531064d623c8922793359cd78f98022ac..9285f992c811a5041476ea65ca55c7072e526825 100644
--- a/mojo/public/tools/bindings/generators/cpp_templates/struct_traits_declaration.tmpl
+++ b/mojo/public/tools/bindings/generators/cpp_templates/struct_traits_declaration.tmpl
@@ -9,9 +9,15 @@ struct {{export_attribute}} StructTraits<{{mojom_type}}::DataView,
 {%- for field in struct.fields %}
 {%-   set return_ref = field.kind|is_object_kind or
                        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 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 %}