cefclient: views: Support window state restore (see issue #3359)

The cefclient sample app will persist window state across application restart
if run with views, cache_path and persist_user_references enabled.

To test:
1. Run `cefclient --use-views --cache-path=/path/to/cache --persist-user-preferences`
2. Move or resize the window, maximize, minimize, etc.
3. Exit cefclient.
4. Run cefclient again with the same arguments. The previous window state will
   be restored.

libcef/browser/views/window_impl.cc

@@ -391,6 +391,9 @@ void CefWindowImpl::OnWindowClosing() {
 #if defined(USE_AURA)
   unhandled_key_event_handler_.reset();
 #endif
+
+  if (delegate())
+    delegate()->OnWindowClosing(this);
 }
 
 void CefWindowImpl::OnWindowViewDeleted() {