mash: Convert system tray logout button to wm common types

ash/mus/bridge/wm_shell_mus.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 "ash/mus/bridge/wm_shell_mus.h"
 
 #include "ash/common/default_accessibility_delegate.h"
 #include "ash/common/display/display_info.h"
 #include "ash/common/session/session_state_delegate.h"
 #include "ash/common/shell_observer.h"
@@ skipping 86 matching lines @@
   WmShell::Set(this);
 
   wm_shell_common_.reset(new WmShellCommon);
   wm_shell_common_->CreateMruWindowTracker();
 
   accessibility_delegate_.reset(new DefaultAccessibilityDelegate);
   SetSystemTrayDelegate(base::WrapUnique(new DefaultSystemTrayDelegate));
 }
 
 WmShellMus::~WmShellMus() {
-  SetSystemTrayDelegate(nullptr);
+  DeleteSystemTrayDelegate();
   DeleteWindowSelectorController();
   wm_shell_common_->DeleteMruWindowTracker();
   RemoveClientObserver();
   WmShell::Set(nullptr);
 }
 
 // static
 WmShellMus* WmShellMus::Get() {
   return static_cast<WmShellMus*>(WmShell::Get());
 }
@@ skipping 195 matching lines @@
                     OnWindowActivated(gained_active, lost_active));
 }
 
 void WmShellMus::OnWillDestroyClient(::mus::WindowTreeClient* client) {
   DCHECK_EQ(client, client_);
   RemoveClientObserver();
 }
 
 }  // namespace mus
 }  // namespace ash