Add support code to test the cast system tray item.

ash/system/tray/system_tray.cc

Index: ash/system/tray/system_tray.cc
diff --git a/ash/system/tray/system_tray.cc b/ash/system/tray/system_tray.cc
index baca0ad2c61e803374d722b02be2f85c958f4867..d23cc7873bc3e4324fcbd2dd984cbc04342aeee5 100644
--- a/ash/system/tray/system_tray.cc
+++ b/ash/system/tray/system_tray.cc
@@ -182,7 +182,8 @@ void SystemTray::CreateItems(SystemTrayDelegate* delegate) {
   AddTrayItem(new TrayVPN(this));
   AddTrayItem(new TraySms(this));
   AddTrayItem(new TrayBluetooth(this));
-  AddTrayItem(new TrayCast(this));
+  tray_cast_ = new TrayCast(this);
+  AddTrayItem(tray_cast_);
   AddTrayItem(new TrayDisplay(this));
   screen_capture_tray_item_ = new ScreenCaptureTrayItem(this);
   AddTrayItem(screen_capture_tray_item_);
@@ -205,7 +206,6 @@ void SystemTray::CreateItems(SystemTrayDelegate* delegate) {
 #elif defined(OS_LINUX)
   AddTrayItem(tray_accessibility_);
   AddTrayItem(new TrayBluetooth(this));
-  AddTrayItem(new TrayCast(this));
   AddTrayItem(new TrayUpdate(this));
   AddTrayItem(tray_date_);
 #endif
@@ -694,6 +694,8 @@ views::View* SystemTray::GetTrayItemViewForTest(SystemTrayItem* item) {
   return it == tray_item_map_.end() ? NULL : it->second;
 }
 
+TrayCast* SystemTray::GetTrayCastForTesting() const { return tray_cast_; }
+
 TrayDate* SystemTray::GetTrayDateForTesting() const { return tray_date_; }
 
 bool SystemTray::PerformAction(const ui::Event& event) {