Mojo C++ bindings: switch part of the services/ui/public/interfaces mojom target to use STL types.

services/ui/ws/user_id_tracker.cc

Index: services/ui/ws/user_id_tracker.cc
diff --git a/services/ui/ws/user_id_tracker.cc b/services/ui/ws/user_id_tracker.cc
index e1f980fabf3a5f71495f59fe1c536f5aa37096cb..4640368823f180d570327661a6496eb4ce4ae5a6 100644
--- a/services/ui/ws/user_id_tracker.cc
+++ b/services/ui/ws/user_id_tracker.cc
@@ -61,7 +61,7 @@ void UserIdTracker::Bind(mojom::UserAccessManagerRequest request) {
   bindings_.AddBinding(this, std::move(request));
 }
 
-void UserIdTracker::SetActiveUser(const mojo::String& user_id) {
+void UserIdTracker::SetActiveUser(const std::string& user_id) {
   if (!IsValidUserId(user_id))
     AddUserId(user_id);
   SetActiveUserId(user_id);