Makes windowserver not send messages during shutdown

services/ui/ws/window_server.cc

Index: services/ui/ws/window_server.cc
diff --git a/services/ui/ws/window_server.cc b/services/ui/ws/window_server.cc
index 3274da7cfa4d750f8bfea8203be64d0c91cd96e0..e02e3c84bb2681f73d44cb28d41ac7cc8d0de2f3 100644
--- a/services/ui/ws/window_server.cc
+++ b/services/ui/ws/window_server.cc
@@ -61,6 +61,9 @@ WindowServer::WindowServer(WindowServerDelegate* delegate)
 WindowServer::~WindowServer() {
   in_destructor_ = true;
 
+  for (auto& pair : tree_map_)
+    pair.second->PrepareForWindowServerShutdown();
+
   // Destroys the window trees results in querying for the display. Tear down
   // the displays first so that the trees are notified of the display going
   // away while the display is still valid.