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

ash/system/audio/tray_volume.cc

Index: ash/system/audio/tray_volume.cc
diff --git a/ash/system/audio/tray_volume.cc b/ash/system/audio/tray_volume.cc
index 72a0e74a011f642f4c830374fb35754ef2c11967..13d2c8eb16041fc0c9934d726a0edd868bd38f10 100644
--- a/ash/system/audio/tray_volume.cc
+++ b/ash/system/audio/tray_volume.cc
@@ -8,6 +8,7 @@
 
 #include "ash/ash_constants.h"
 #include "ash/shell.h"
+#include "ash/system/tray/system_tray.h"
 #include "ash/system/tray/system_tray_delegate.h"
 #include "ash/system/tray/tray_constants.h"
 #include "ash/system/tray/tray_views.h"
@@ -213,8 +214,8 @@ class VolumeView : public views::View,
 
 }  // namespace tray
 
-TrayVolume::TrayVolume()
-    : TrayImageItem(IDR_AURA_UBER_TRAY_VOLUME_MUTE),
+TrayVolume::TrayVolume(SystemTray* system_tray)
+    : TrayImageItem(system_tray, IDR_AURA_UBER_TRAY_VOLUME_MUTE),
       volume_view_(NULL),
       is_default_view_(false) {
 }