ash/mus: Fix a crash during tear down.

ash/mus/window_manager.cc

Index: ash/mus/window_manager.cc
diff --git a/ash/mus/window_manager.cc b/ash/mus/window_manager.cc
index a0126dba54709d6cca30f958e72029177a585262..964a1dfeaa85851962ed4bc7231afc88b55bc18e 100644
--- a/ash/mus/window_manager.cc
+++ b/ash/mus/window_manager.cc
@@ -214,14 +214,16 @@ void WindowManager::OnDidDestroyClient(ui::WindowTreeClient* client) {
   // |root_window_controllers_|.
   DCHECK(root_window_controllers_.empty());
 
+  // Observers can rely on WmShell from the callback. So notify the observers
+  // before destroying it.
+  FOR_EACH_OBSERVER(WindowManagerObserver, observers_,
+                    OnWindowTreeClientDestroyed());
+
   lookup_.reset();
   shell_->Shutdown();
   shell_.reset();
   shadow_controller_.reset();
 
-  FOR_EACH_OBSERVER(WindowManagerObserver, observers_,
-                    OnWindowTreeClientDestroyed());
-
   pointer_watcher_event_router_.reset();
 
   window_tree_client_ = nullptr;