Makes WindowTree::DeleteWindow() unembed for roots

services/ui/ws/server_window_observer.h

Index: services/ui/ws/server_window_observer.h
diff --git a/services/ui/ws/server_window_observer.h b/services/ui/ws/server_window_observer.h
index a173170a6db5fd6336c4fcc659cfaf586c4997b1..7ffae1f1c82540121402a20f951d9348168f1b56 100644
--- a/services/ui/ws/server_window_observer.h
+++ b/services/ui/ws/server_window_observer.h
@@ -78,6 +78,9 @@ class ServerWindowObserver {
       const std::string& name,
       const std::vector<uint8_t>* new_data) {}
 
+  // Called when the window is no longer an embed root.
+  virtual void OnWindowEmbeddedAppDisconnected(ServerWindow* window) {}
+
   // Called when a transient child is added to |window|.
   virtual void OnTransientWindowAdded(ServerWindow* window,
                                       ServerWindow* transient_child) {}