Fix the regression caused by the refactoring.

I accidentally delete one line code that should not be deleted. Bring it
back.

Bug: 887042
Change-Id: I9a6c4131eb2003888e0edd987ae7362ad847c568
Reviewed-on: https://chromium-review.googlesource.com/1247182Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Xiaoqian Dai <xdai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594508}

ash/wm/tablet_mode/tablet_mode_controller.cc

@@ -254,6 +254,9 @@ void TabletModeController::OnShellInitialized() {
     Shell::Get()->window_tree_host_manager()->AddObserver(this);
     chromeos::AccelerometerReader::GetInstance()->AddObserver(this);
     ui::InputDeviceManager::GetInstance()->AddObserver(this);
+    bluetooth_devices_observer_ = std::make_unique<BluetoothDevicesObserver>(
+        base::BindRepeating(&TabletModeController::UpdateBluetoothDevice,
+                            base::Unretained(this)));
 
     chromeos::PowerManagerClient* power_manager_client =
         chromeos::DBusThreadManager::Get()->GetPowerManagerClient();