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..6dd9404e5ba2bb91ca127ff2cbc0618667bb9172 100644
--- a/ash/mus/window_manager.cc
+++ b/ash/mus/window_manager.cc
@@ -214,14 +214,14 @@ void WindowManager::OnDidDestroyClient(ui::WindowTreeClient* client) {
   // |root_window_controllers_|.
   DCHECK(root_window_controllers_.empty());
 
+  FOR_EACH_OBSERVER(WindowManagerObserver, observers_,

[James Cook, 2016/08/22 15:46:40]

Can you add a comment here saying this must be destroyed before |shell_|,
something like your CL description?

(We've found such comments in ash/shell.cc to be invaluable when sorting out
init/shutdown order.)

[sadrul, 2016/08/22 15:52:20]

Done. I looked into writing a unit-test for this, but doesn't like we have those
at the moment (we have some shell-tests in window_manager_unittest.cc, but I
don't think we can test things like this there).

+                    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;