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

services/ui/ws/user_id_tracker.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef SERVICES_UI_WS_USER_ID_TRACKER_H_
 #define SERVICES_UI_WS_USER_ID_TRACKER_H_
 
 #include <stdint.h>
 
 #include <set>
@@ skipping 26 matching lines @@
 
   void AddObserver(UserIdTrackerObserver* observer);
   void RemoveObserver(UserIdTrackerObserver* observer);
 
   void Bind(mojom::UserAccessManagerRequest request);
 
  private:
   using UserIdSet = std::set<UserId>;
 
   // mojom::UserAccessManager:
-  void SetActiveUser(const mojo::String& user_id) override;
+  void SetActiveUser(const std::string& user_id) override;
 
   base::ObserverList<UserIdTrackerObserver> observers_;
 
   UserIdSet ids_;
   UserId active_id_;
 
   mojo::BindingSet<mojom::UserAccessManager> bindings_;
 
   DISALLOW_COPY_AND_ASSIGN(UserIdTracker);
 };
 
 }  // namespace ws
 }  // namespace ui
 
 #endif  // SERVICES_UI_WS_USER_ID_TRACKER_H_