mus: aura::Env should be destroyed last in the mus client.

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/focus_client.h"
 #include "ui/aura/env.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), env_(Env::GetInstance()) {
-  DCHECK(env_);
-  env_->AddObserver(this);
+    : delegate_(delegate), window_tree_(window_tree) {
+  Env::GetInstance()->AddObserver(this);
 }
 
 FocusSynchronizer::~FocusSynchronizer() {
-  if (env_) {
-    SetActiveFocusClient(nullptr);
-    env_->RemoveObserver(this);
-  }
+  SetActiveFocusClient(nullptr);
+  Env::GetInstance()->RemoveObserver(this);
 }
 
 void FocusSynchronizer::SetFocusFromServer(WindowMus* window) {
   if (focused_window_ == window)
     return;
 
   DCHECK(!setting_focus_);
   base::AutoReset<bool> focus_reset(&setting_focus_, true);
   base::AutoReset<WindowMus*> window_setting_focus_to_reset(
       &window_setting_focus_to_, window);
@@ skipping 38 matching lines @@
   WindowMus* gained_focus_mus = WindowMus::Get(gained_focus);
   if (setting_focus_ && gained_focus_mus == window_setting_focus_to_) {
     focused_window_ = gained_focus_mus;
     return;
   }
   SetFocusedWindow(gained_focus_mus);
 }
 
 void FocusSynchronizer::OnWindowInitialized(Window* window) {}
 
-void FocusSynchronizer::OnWillDestroyEnv() {
-  // TODO(sky): this override should not be necessary. http://crbug.com/674803.
-  SetActiveFocusClient(nullptr);
-  env_->RemoveObserver(this);
-  env_ = nullptr;
-}
-
 void FocusSynchronizer::OnActiveFocusClientChanged(
     client::FocusClient* focus_client,
     Window* window) {
   SetActiveFocusClient(focus_client);
   if (setting_focus_)
     return;
 
   if (focus_client) {
     Window* focused_window = focus_client->GetFocusedWindow();
     SetFocusedWindow(focused_window ? WindowMus::Get(focused_window)
                                     : WindowMus::Get(window));
   } else {
     SetFocusedWindow(nullptr);
   }
 }
 
 }  // namespace aura