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

ash/system/bluetooth/tray_bluetooth.h

Index: ash/system/bluetooth/tray_bluetooth.h
diff --git a/ash/system/bluetooth/tray_bluetooth.h b/ash/system/bluetooth/tray_bluetooth.h
index 2ae655b0b4e1723f4464ce40d16617daac9f21db..20509c863f6ffe0add8959d43ebb200bb709a170 100644
--- a/ash/system/bluetooth/tray_bluetooth.h
+++ b/ash/system/bluetooth/tray_bluetooth.h
@@ -9,6 +9,9 @@
 #include "ash/system/tray/system_tray_item.h"
 
 namespace ash {
+
+class SystemTray;
+
 namespace internal {
 
 namespace tray {
@@ -19,7 +22,7 @@ class BluetoothDetailedView;
 class TrayBluetooth : public SystemTrayItem,
                       public BluetoothObserver {
  public:
-  TrayBluetooth();
+  explicit TrayBluetooth(SystemTray* system_tray);
   virtual ~TrayBluetooth();
 
  private: