mus: Make sure only valid user-ids can be active.

components/mus/ws/user_id_tracker.cc

 // 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.
 
 #include "components/mus/ws/user_id_tracker.h"
 
 #include "components/mus/ws/user_id_tracker_observer.h"
 #include "services/shell/public/interfaces/connector.mojom.h"
 
 namespace mus {
 namespace ws {
 
 UserIdTracker::UserIdTracker() : active_id_(shell::mojom::kRootUserID) {
   ids_.insert(active_id_);
 }
+
 UserIdTracker::~UserIdTracker() {
 }
 
 bool UserIdTracker::IsValidUserId(const UserId& id) const {
   return ids_.count(id) > 0;
 }
 
 void UserIdTracker::SetActiveUserId(const UserId& id) {
+  DCHECK(IsValidUserId(id));
   if (id == active_id_)
     return;
 
   const UserId previously_active_id = active_id_;
   active_id_ = id;
   FOR_EACH_OBSERVER(UserIdTrackerObserver, observers_,
                     OnActiveUserIdChanged(previously_active_id, id));
 }
 
 void UserIdTracker::AddUserId(const UserId& id) {
@@ skipping 23 matching lines @@
 }
 
 void UserIdTracker::SetActiveUser(const mojo::String& user_id) {
   if (!IsValidUserId(user_id))
     AddUserId(user_id);
   SetActiveUserId(user_id);
 }
 
 }  // namespace ws
 }  // namespace mus