Change userid from a uint32_t to a string guid

components/mus/ws/user_id_tracker_observer.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 COMPONENTS_MUS_WS_USER_ID_TRACKER_OBSERVER_H_
 #define COMPONENTS_MUS_WS_USER_ID_TRACKER_OBSERVER_H_
 
 #include <stdint.h>
 
 #include "components/mus/ws/user_id.h"
 
 namespace mus {
 namespace ws {
 
 class UserIdTrackerObserver {
  public:
-  virtual void OnActiveUserIdChanged(UserId id) = 0;
-  virtual void OnUserIdAdded(UserId id) = 0;
-  virtual void OnUserIdRemoved(UserId id) = 0;
+  virtual void OnActiveUserIdChanged(const UserId& id) = 0;
+  virtual void OnUserIdAdded(const UserId& id) = 0;
+  virtual void OnUserIdRemoved(const UserId& id) = 0;
 
  protected:
   virtual ~UserIdTrackerObserver() {}
 };
 
 }  // namespace ws
 }  // namespace mus
 
 #endif  // COMPONENTS_MUS_WS_USER_ID_TRACKER_OBSERVER_H_