Have WindowTreeClient detect error state earlier

ui/aura/mus/window_tree_client_observer.h

Index: ui/aura/mus/window_tree_client_observer.h
diff --git a/ui/aura/mus/window_tree_client_observer.h b/ui/aura/mus/window_tree_client_observer.h
index 7d108059eb78eea00c8bbdfa50bb8f159edfd347..7410549a66ffb8dfc07bbcfefd0ae649ccb6ffdb 100644
--- a/ui/aura/mus/window_tree_client_observer.h
+++ b/ui/aura/mus/window_tree_client_observer.h
@@ -13,14 +13,9 @@ 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() {}
 };