chromeos: makes it possible for windows to outlive WindowTreeClient

ui/aura/mus/focus_synchronizer.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 "ui/aura/mus/focus_synchronizer.h"
 
 #include "base/auto_reset.h"
 #include "services/ui/public/interfaces/window_tree.mojom.h"
 #include "ui/aura/client/aura_constants.h"
 #include "ui/aura/client/focus_client.h"
 #include "ui/aura/mus/focus_synchronizer_delegate.h"
 #include "ui/aura/mus/window_mus.h"
 #include "ui/aura/window.h"
 
 namespace aura {
 
 FocusSynchronizer::FocusSynchronizer(FocusSynchronizerDelegate* delegate,
                                      ui::mojom::WindowTree* window_tree)
     : delegate_(delegate), window_tree_(window_tree) {}
 
 FocusSynchronizer::~FocusSynchronizer() {
   SetActiveFocusClientInternal(nullptr);
+  if (active_focus_client_root_)
+    active_focus_client_root_->RemoveObserver(this);
 }
 
 void FocusSynchronizer::AddObserver(FocusSynchronizerObserver* observer) {
   observers_.AddObserver(observer);
 }
 
 void FocusSynchronizer::RemoveObserver(FocusSynchronizerObserver* observer) {
   observers_.RemoveObserver(observer);
 }
 
@@ skipping 108 matching lines @@
                                                 intptr_t old) {
   DCHECK(!is_singleton_focus_client_);
   if (key != client::kFocusClientKey)
     return;
 
   // Assume if the focus client changes the window is being destroyed.
   SetActiveFocusClient(nullptr, nullptr);
 }
 
 }  // namespace aura