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

ash/system/audio/tray_audio.cc

Index: ash/system/audio/tray_audio.cc
diff --git a/ash/system/audio/tray_audio.cc b/ash/system/audio/tray_audio.cc
index 332dd44d79f4df216e0d920d52113ad43e236038..975d692dfe21e3d68d46d78cfcda9c80bf28286d 100644
--- a/ash/system/audio/tray_audio.cc
+++ b/ash/system/audio/tray_audio.cc
@@ -48,11 +48,11 @@ TrayAudio::TrayAudio(SystemTray* system_tray,
       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);
 }