A11y: Add a browser test of TrayAccessibility.

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 d225ce056ab0589a5577da55b18d52121a851a44..66688cfe8ede3d56168b4ba614ec3695a6542d26 100644
--- a/ash/system/tray/system_tray.cc
+++ b/ash/system/tray/system_tray.cc
@@ -131,7 +131,8 @@ void SystemTray::CreateItems(SystemTrayDelegate* delegate) {
   AddTrayItem(new internal::TrayLogoutButton(this));
   AddTrayItem(new internal::TrayUser(this));
   AddTrayItem(new internal::TrayIME(this));
-  AddTrayItem(new internal::TrayAccessibility(this));
+  tray_accessibility_ = new internal::TrayAccessibility(this);
+  AddTrayItem(tray_accessibility_);
   AddTrayItem(new internal::TrayPower(this));
 #if defined(OS_CHROMEOS)
   AddTrayItem(new internal::TrayNetwork(this));