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

ash/system/user/tray_user.cc

Index: ash/system/user/tray_user.cc
diff --git a/ash/system/user/tray_user.cc b/ash/system/user/tray_user.cc
index e476254333c4c6e0ae373362d5ae29924b601679..78ca43b0b2e2b8d520ff8364b73ac719e9bb76a8 100644
--- a/ash/system/user/tray_user.cc
+++ b/ash/system/user/tray_user.cc
@@ -5,6 +5,7 @@
 #include "ash/system/user/tray_user.h"
 
 #include "ash/shell.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_item_view.h"
@@ -246,8 +247,9 @@ class UserView : public views::View,
 
 }  // namespace tray
 
-TrayUser::TrayUser()
-    : user_(NULL),
+TrayUser::TrayUser(SystemTray* system_tray)
+    : SystemTrayItem(system_tray),
+      user_(NULL),
       avatar_(NULL),
       label_(NULL) {
 }