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

ash/system/logout_button/tray_logout_button.h

Index: ash/system/logout_button/tray_logout_button.h
diff --git a/ash/system/logout_button/tray_logout_button.h b/ash/system/logout_button/tray_logout_button.h
index ca51d9ddbb0e23bd305f22d9e3bf6030ae65b8d5..7483c62b154885607b0f372edd670dc2e54c720d 100644
--- a/ash/system/logout_button/tray_logout_button.h
+++ b/ash/system/logout_button/tray_logout_button.h
@@ -11,6 +11,9 @@
 #include "base/compiler_specific.h"
 
 namespace ash {
+
+class SystemTray;
+
 namespace internal {
 
 namespace tray {
@@ -21,7 +24,7 @@ class LogoutButton;
 // kShowLogoutButtonInTray pref.
 class TrayLogoutButton : public SystemTrayItem, public LogoutButtonObserver {
  public:
-  TrayLogoutButton();
+  explicit TrayLogoutButton(SystemTray* system_tray);
 
   // Overridden from SystemTrayItem.
   virtual views::View* CreateTrayView(user::LoginStatus status) OVERRIDE;