Have WindowTreeClient detect error state earlier

ui/aura/mus/window_tree_client_observer.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef UI_AURA_MUS_WINDOW_TREE_CLIENT_OBSERVER_H_
 #define UI_AURA_MUS_WINDOW_TREE_CLIENT_OBSERVER_H_
 
 #include "ui/aura/aura_export.h"
 
 namespace aura {
 
 class WindowTreeClient;
 
 class AURA_EXPORT WindowTreeClientObserver {
  public:
-  // Called early on in the destructor of WindowTreeClient; before any windows
-  // have been destroyed.
+  // Called early on in the destructor of WindowTreeClient.
   virtual void OnWillDestroyClient(WindowTreeClient* client) {}
 
-  // Called at the end of WindowTreeClient's destructor. At this point
-  // observers should drop all references to |client|.
-  virtual void OnDidDestroyClient(WindowTreeClient* client) {}
-
  protected:
   virtual ~WindowTreeClientObserver() {}
 };
 
 }  // namespace aura
 
 #endif  // UI_AURA_MUS_WINDOW_TREE_CLIENT_OBSERVER_H_