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

ash/system/tray/system_tray_item.h

Index: ash/system/tray/system_tray_item.h
diff --git a/ash/system/tray/system_tray_item.h b/ash/system/tray/system_tray_item.h
index 6361529f8448798a6063310228fa3a620f136eb6..72c1aea15e8c635cf4e3503604e92a32507d6a76 100644
--- a/ash/system/tray/system_tray_item.h
+++ b/ash/system/tray/system_tray_item.h
@@ -17,13 +17,15 @@ class View;
 
 namespace ash {
 
+class SystemTray;
+
 namespace internal {
 class TrayItemView;
 }
 
 class ASH_EXPORT SystemTrayItem {
  public:
-  SystemTrayItem();
+  explicit SystemTrayItem(SystemTray* system_tray);
   virtual ~SystemTrayItem();
 
   // Create* functions may return NULL if nothing should be displayed for the
@@ -100,7 +102,12 @@ class ASH_EXPORT SystemTrayItem {
   // the launcher is in the auto-hide state. Default is true.
   virtual bool ShouldShowLauncher() const;
 
+  // Returns the system tray that this item belongs to.
+  SystemTray* system_tray() const { return system_tray_; }
+
  private:
+  SystemTray* system_tray_;
+
   DISALLOW_COPY_AND_ASSIGN(SystemTrayItem);
 };