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

ash/system/chromeos/network/tray_vpn.h

Index: ash/system/chromeos/network/tray_vpn.h
diff --git a/ash/system/chromeos/network/tray_vpn.h b/ash/system/chromeos/network/tray_vpn.h
index 1e675d535769dea677c4738cc195cca1603c6fe4..3a250b3154da931db9581d6da25af1a19a9f225e 100644
--- a/ash/system/chromeos/network/tray_vpn.h
+++ b/ash/system/chromeos/network/tray_vpn.h
@@ -10,6 +10,9 @@
 #include "base/memory/scoped_ptr.h"
 
 namespace ash {
+
+class SystemTray;
+
 namespace internal {
 
 namespace tray {
@@ -21,7 +24,7 @@ class VpnDetailedView;
 class TrayVPN : public SystemTrayItem,
                 public NetworkObserver {
  public:
-  TrayVPN();
+  explicit TrayVPN(SystemTray* system_tray);
   virtual ~TrayVPN();
 
   // Overridden from SystemTrayItem.