mash: Introduce WmSystemTrayNotifier for //ash/common

ash/common/wm_shell.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/common/wm_shell.h"
 
 #include "ash/common/system/tray/system_tray_delegate.h"
+#include "ash/common/system/tray/wm_system_tray_notifier.h"
 #include "base/logging.h"
 
 namespace ash {
 
 // static
 WmShell* WmShell::instance_ = nullptr;
 
 // static
 void WmShell::Set(WmShell* instance) {
   instance_ = instance;
 }
 
 // static
 WmShell* WmShell::Get() {
   return instance_;
 }
 
-WmShell::WmShell() {}
+WmShell::WmShell() : system_tray_notifier_(new WmSystemTrayNotifier) {}
 
 WmShell::~WmShell() {}
 
 void WmShell::SetSystemTrayDelegate(
     std::unique_ptr<SystemTrayDelegate> delegate) {
   if (delegate) {
     DCHECK(!system_tray_delegate_);
     // TODO(jamescook): Create via ShellDelegate once it moves to //ash/common.
     system_tray_delegate_ = std::move(delegate);
     system_tray_delegate_->Initialize();
   } else {
     DCHECK(system_tray_delegate_);
     system_tray_delegate_->Shutdown();
     system_tray_delegate_.reset();
   }
 }
 
 }  // namespace ash