Make the control buttons on a notification always show on non Chrome OS platform.

ui/arc/notification/arc_custom_notification_view.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/arc/notification/arc_custom_notification_view.h"
 
 #include "ash/wm/window_util.h"
 #include "base/auto_reset.h"
 #include "base/memory/ptr_util.h"
 #include "components/exo/notification_surface.h"
@@ skipping 223 matching lines @@
 void ArcCustomNotificationView::ControlButton::OnBlur() {
   message_center::PaddedButton::OnBlur();
   owner_->UpdateControlButtonsVisibility();
 }
 
 ArcCustomNotificationView::ArcCustomNotificationView(ArcNotificationItem* item)
     : item_(item),
       notification_key_(item->GetNotificationKey()),
       event_forwarder_(new EventForwarder(this)) {
   SetFocusBehavior(FocusBehavior::ALWAYS);
+  set_notify_enter_exit_on_child(true);
 
   item_->IncrementWindowRefCount();
   item_->AddObserver(this);
 
   auto* surface_manager = ArcNotificationSurfaceManager::Get();
   if (surface_manager) {
     surface_manager->AddObserver(this);
     exo::NotificationSurface* surface =
         surface_manager->GetSurface(notification_key_);
     if (surface)
@@ skipping 475 matching lines @@
     }
     if (close_button_) {
       close_button_->set_background(
           views::Background::CreateSolidBackground(current_color));
       close_button_->SchedulePaint();
     }
   }
 }
 
 }  // namespace arc