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

ash/system/settings/tray_settings.h

Index: ash/system/settings/tray_settings.h
diff --git a/ash/system/settings/tray_settings.h b/ash/system/settings/tray_settings.h
index 9bfeca3ffdd31ffbed1bc5e4d03d908f75aaaf3b..2754f369b322478cae4abcaf04d9b1e15a501507 100644
--- a/ash/system/settings/tray_settings.h
+++ b/ash/system/settings/tray_settings.h
@@ -9,6 +9,9 @@
 #include "ash/system/tray/system_tray_item.h"
 
 namespace ash {
+
+class SystemTray;
+
 namespace internal {
 
 namespace tray {
@@ -18,7 +21,7 @@ class SettingsDefaultView;
 class TraySettings : public SystemTrayItem,
                      public PowerStatusObserver {
  public:
-  TraySettings();
+  explicit TraySettings(SystemTray* system_tray);
   virtual ~TraySettings();
 
  private: