Added Ash.SystemMenu.DefaultView.VisibleItems histogram.

ash/common/system/chromeos/bluetooth/tray_bluetooth.cc

Index: ash/common/system/chromeos/bluetooth/tray_bluetooth.cc
diff --git a/ash/common/system/chromeos/bluetooth/tray_bluetooth.cc b/ash/common/system/chromeos/bluetooth/tray_bluetooth.cc
index 482d4f09edc7044ca55ae146e732ddb987cf3472..16d683f6a84c7d5ea11b5580a785091df937e2b2 100644
--- a/ash/common/system/chromeos/bluetooth/tray_bluetooth.cc
+++ b/ash/common/system/chromeos/bluetooth/tray_bluetooth.cc
@@ -394,7 +394,9 @@ class BluetoothDetailedView : public TrayDetailsView,
 }  // namespace tray
 
 TrayBluetooth::TrayBluetooth(SystemTray* system_tray)
-    : SystemTrayItem(system_tray), default_(NULL), detailed_(NULL) {
+    : SystemTrayItem(system_tray, UMA_BLUETOOTH),
+      default_(nullptr),
+      detailed_(nullptr) {
   WmShell::Get()->system_tray_notifier()->AddBluetoothObserver(this);
 }