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

ash/system/brightness/tray_brightness.h

Index: ash/system/brightness/tray_brightness.h
diff --git a/ash/system/brightness/tray_brightness.h b/ash/system/brightness/tray_brightness.h
index 6fc658f5c357cb1a11c225612ee539b3c994a4a4..aae50c036f72ec2a0c1e6f196551ff3957610dc2 100644
--- a/ash/system/brightness/tray_brightness.h
+++ b/ash/system/brightness/tray_brightness.h
@@ -11,6 +11,9 @@
 #include "base/memory/weak_ptr.h"
 
 namespace ash {
+
+class SystemTray;
+
 namespace internal {
 
 namespace tray {
@@ -20,7 +23,7 @@ class BrightnessView;
 class TrayBrightness : public SystemTrayItem,
                        public BrightnessObserver {
  public:
-  TrayBrightness();
+  explicit TrayBrightness(SystemTray* system_tray);
   virtual ~TrayBrightness();
 
  private: