Add a row in the network tray to inform users to turn Bluetooth on to enable Tether.

ash/system/bluetooth/tray_bluetooth.cc

Index: ash/system/bluetooth/tray_bluetooth.cc
diff --git a/ash/system/bluetooth/tray_bluetooth.cc b/ash/system/bluetooth/tray_bluetooth.cc
index 3a49c9898f79c2d7a324e71f259fb2771b947eef..13a0f69e398ea288483106498161dd091133f9f5 100644
--- a/ash/system/bluetooth/tray_bluetooth.cc
+++ b/ash/system/bluetooth/tray_bluetooth.cc
@@ -314,8 +314,8 @@ class BluetoothDetailedView : public TrayDetailsView {
 
     // Show user Bluetooth state if there is no bluetooth devices in list.
     if (device_map_.size() == 0 && bluetooth_available && bluetooth_enabled) {
-      scroll_content()->AddChildView(
-          new InfoLabel(IDS_ASH_STATUS_TRAY_BLUETOOTH_DISCOVERING));
+      scroll_content()->AddChildView(TrayDetailsView::CreateInfoLabel(
+          IDS_ASH_STATUS_TRAY_BLUETOOTH_DISCOVERING));
     }
 
     // Focus the device which was focused before the device-list update.