Stop using shell::GetInstance()->system_tray() in system tray items

ash/system/chromeos/tray_display.h

Index: ash/system/chromeos/tray_display.h
diff --git a/ash/system/chromeos/tray_display.h b/ash/system/chromeos/tray_display.h
index 0685a9de29dc3c50e2b5e44f3c86256a80ad8532..d471df876c0d15cf27e44bc5d5cb60aa46175fe4 100644
--- a/ash/system/chromeos/tray_display.h
+++ b/ash/system/chromeos/tray_display.h
@@ -16,7 +16,6 @@ class View;
 }
 
 namespace ash {
-
 namespace internal {
 class DisplayView;
 
@@ -24,7 +23,7 @@ class TrayDisplay : public SystemTrayItem,
                     public chromeos::OutputConfigurator::Observer,
                     public gfx::DisplayObserver {
  public:
-  TrayDisplay();
+  explicit TrayDisplay(SystemTray* system_tray);
   virtual ~TrayDisplay();
 
  private: