arc: Try to restart ARC++ kiosk app if the task was closed.

As ARC++ kiosk should always be running during the ARC++ kiosk session,
it make sense to restart it when there is notification that the task was closed.
However, it won't restart the app if some preconditions are not yet satisfied,
e.g. policy is not compliant, app is not ready.

BUG=676825
TEST=Manual, Run Kiosk on ARC++ device

Review-Url: https://codereview.chromium.org/2599333002
Cr-Commit-Position: refs/heads/master@{#441932}

chrome/browser/chromeos/app_mode/arc/arc_kiosk_app_service.cc

@@ -63,6 +63,9 @@ void ArcKioskAppService::OnTaskDestroyed(int32_t task_id) {
   if (task_id == task_id_) {
     app_launcher_.reset();
     task_id_ = -1;
+    // Trying to restart app if it was somehow closed or crashed
+    // as kiosk app should always be running during the session.
+    PreconditionsChanged();
   }
 }