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

ash/system/logout_button/tray_logout_button.cc

Index: ash/system/logout_button/tray_logout_button.cc
diff --git a/ash/system/logout_button/tray_logout_button.cc b/ash/system/logout_button/tray_logout_button.cc
index 896f6016e841e4a4cdae7fc92c2ed0806995f272..c609190d4d98a6b76336ebe37f6b854a0e7343ef 100644
--- a/ash/system/logout_button/tray_logout_button.cc
+++ b/ash/system/logout_button/tray_logout_button.cc
@@ -7,6 +7,7 @@
 #include <cstddef>
 
 #include "ash/shell.h"
+#include "ash/system/tray/system_tray.h"
 #include "ash/system/tray/system_tray_delegate.h"
 #include "ash/system/user/login_status.h"
 #include "base/logging.h"
@@ -116,7 +117,9 @@ class LogoutButton : public views::LabelButton,
 
 }  // namespace tray
 
-TrayLogoutButton::TrayLogoutButton() : logout_button_(NULL) {
+TrayLogoutButton::TrayLogoutButton(SystemTray* system_tray)
+    : SystemTrayItem(system_tray),
+      logout_button_(NULL) {
 }
 
 views::View* TrayLogoutButton::CreateTrayView(user::LoginStatus status) {