Added Ash.SystemMenu.DefaultView.VisibleItems histogram.

ash/common/system/chromeos/network/tray_vpn.cc

Index: ash/common/system/chromeos/network/tray_vpn.cc
diff --git a/ash/common/system/chromeos/network/tray_vpn.cc b/ash/common/system/chromeos/network/tray_vpn.cc
index ac80f521058cbdfc87046f183b9f79436a681fb6..bb797c6304df76891a9c252566ef92c2292cef38 100644
--- a/ash/common/system/chromeos/network/tray_vpn.cc
+++ b/ash/common/system/chromeos/network/tray_vpn.cc
@@ -112,7 +112,9 @@ class VpnDefaultView : public TrayItemMore,
 }  // namespace tray
 
 TrayVPN::TrayVPN(SystemTray* system_tray)
-    : SystemTrayItem(system_tray), default_(NULL), detailed_(NULL) {
+    : SystemTrayItem(system_tray, UMA_VPN),
+      default_(nullptr),
+      detailed_(nullptr) {
   network_state_observer_.reset(new TrayNetworkStateObserver(this));
 }