fix a crash on exiting tablet mode

Bug: 831423
Test: None
Change-Id: If8e8a2e88d009dfea258b79f6b914444f28816ee
Reviewed-on: https://chromium-review.googlesource.com/1024735
Reviewed-by: Dan Erat <derat@chromium.org>
Commit-Queue: Jun Mukai <mukai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#552948}

ash/touch/touch_devices_controller.cc

@@ -191,8 +191,8 @@ void TouchDevicesController::UpdateTouchpadEnabled() {
   if (!GetInputDeviceControllerClient())
     return;  // Happens in tests.
 
-  bool enabled = global_touchpad_enabled_ &&
-                 GetActivePrefService()->GetBoolean(prefs::kTouchpadEnabled);
+  bool enabled = GetTouchpadEnabled(TouchDeviceEnabledSource::GLOBAL) &&
+                 GetTouchpadEnabled(TouchDeviceEnabledSource::USER_PREF);
 
   GetInputDeviceControllerClient()->SetInternalTouchpadEnabled(
       enabled, base::BindRepeating(&OnSetTouchpadEnabledDone, enabled));