Remove now-unnecessary wrappers around gfx::Screen::GetScreen()

ash/system/audio/tray_audio.cc

 // Copyright 2014 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/system/audio/tray_audio.h"
 
 #include <cmath>
 #include <utility>
 
 #include "ash/ash_constants.h"
@@ skipping 30 matching lines @@
 
 namespace ash {
 
 TrayAudio::TrayAudio(SystemTray* system_tray,
                      scoped_ptr<system::TrayAudioDelegate> audio_delegate)
     : TrayImageItem(system_tray, IDR_AURA_UBER_TRAY_VOLUME_MUTE),
       audio_delegate_(std::move(audio_delegate)),
       volume_view_(NULL),
       pop_up_volume_view_(false) {
   Shell::GetInstance()->system_tray_notifier()->AddAudioObserver(this);
-  Shell::GetScreen()->AddObserver(this);
+  gfx::Screen::GetScreen()->AddObserver(this);
 }
 
 TrayAudio::~TrayAudio() {
-  Shell::GetScreen()->RemoveObserver(this);
+  gfx::Screen::GetScreen()->RemoveObserver(this);
   Shell::GetInstance()->system_tray_notifier()->RemoveAudioObserver(this);
 }
 
 // static
 bool TrayAudio::ShowAudioDeviceMenu() {
 #if defined(OS_CHROMEOS)
   return true;
 #else
   return false;
 #endif
@@ skipping 113 matching lines @@
   if (tray_view())
       tray_view()->SetVisible(GetInitialVisibility());
   if (volume_view_) {
     volume_view_->SetVolumeLevel(
         static_cast<float>(audio_delegate_->GetOutputVolumeLevel()) / 100.0f);
     volume_view_->Update();
   }
 }
 
 }  // namespace ash