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

ash/system/tray/tray_empty.cc

Index: ash/system/tray/tray_empty.cc
diff --git a/ash/system/tray/tray_empty.cc b/ash/system/tray/tray_empty.cc
index 670618d42fe67d56b985ab03726acd48b2e10715..eba3ae8547f0bc0186769287aaf7f83b575dc5ea 100644
--- a/ash/system/tray/tray_empty.cc
+++ b/ash/system/tray/tray_empty.cc
@@ -4,6 +4,7 @@
 
 #include "ash/system/tray/tray_empty.h"
 
+#include "ash/system/tray/system_tray.h"
 #include "ui/views/layout/box_layout.h"
 #include "ui/views/background.h"
 #include "ui/views/view.h"
@@ -27,7 +28,9 @@ class EmptyBackground : public views::Background {
 namespace ash {
 namespace internal {
 
-TrayEmpty::TrayEmpty() {}
+TrayEmpty::TrayEmpty(SystemTray* system_tray)
+    : SystemTrayItem(system_tray) {
+}
 
 TrayEmpty::~TrayEmpty() {}