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

ash/system/power/tray_power.cc

Index: ash/system/power/tray_power.cc
diff --git a/ash/system/power/tray_power.cc b/ash/system/power/tray_power.cc
index 27696e70066e853f39e3eb288d678ea0bc7c31bc..996d13246d424671c838095799fb36bb1b086989 100644
--- a/ash/system/power/tray_power.cc
+++ b/ash/system/power/tray_power.cc
@@ -9,6 +9,7 @@
 #include "ash/system/date/date_view.h"
 #include "ash/system/power/power_status_view.h"
 #include "ash/system/power/power_supply_status.h"
+#include "ash/system/tray/system_tray.h"
 #include "ash/system/tray/system_tray_delegate.h"
 #include "ash/system/tray/tray_constants.h"
 #include "ash/system/tray/tray_notification_view.h"
@@ -133,8 +134,9 @@ class PowerNotificationView : public TrayNotificationView {
 
 using tray::PowerNotificationView;
 
-TrayPower::TrayPower()
-    : power_tray_(NULL),
+TrayPower::TrayPower(SystemTray* system_tray)
+    : SystemTrayItem(system_tray),
+      power_tray_(NULL),
       notification_view_(NULL),
       notification_state_(NOTIFICATION_NONE) {
 }