Fixes shutdown race in mash

ash/mus/window_manager_observer.h

Index: ash/mus/window_manager_observer.h
diff --git a/ash/mus/root_windows_observer.h b/ash/mus/window_manager_observer.h
similarity index 51%
rename from ash/mus/root_windows_observer.h
rename to ash/mus/window_manager_observer.h
index 9ef27e5c833922289f88f70e453f2e48f3b49f8e..0fa0982aaaad77ada6cecd37737cb3c08a088f1e 100644
--- a/ash/mus/root_windows_observer.h
+++ b/ash/mus/window_manager_observer.h
@@ -2,28 +2,37 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#ifndef ASH_MUS_ROOT_WINDOW_OBSERVERS_H_
-#define ASH_MUS_ROOT_WINDOW_OBSERVERS_H_
+#ifndef ASH_MUS_WINDOW_OBSERVER_H_
+#define ASH_MUS_WINDOW_OBSERVER_H_
+
+#include <stdint.h>
+
+namespace ui {
+class Event;
+}
 
 namespace ash {
 namespace mus {
 
 class RootWindowController;
 
-class RootWindowsObserver {
+class WindowManagerObserver {
  public:
-  // Called once a new display has been created and the root Window obtained.
-  virtual void OnRootWindowControllerAdded(RootWindowController* controller) {}
+  // Called when the WindowTreeClient associated with the WindowManager is
+  // about to be destroyed.
+  virtual void OnWindowTreeClientDestroyed() {}
 
-  // Called before a RootWindowController is destroyed.
+  virtual void OnAccelerator(uint32_t id, const ui::Event& event) {}
+
+  virtual void OnRootWindowControllerAdded(RootWindowController* controller) {}
   virtual void OnWillDestroyRootWindowController(
       RootWindowController* controller) {}
 
  protected:
-  ~RootWindowsObserver() {}
+  virtual ~WindowManagerObserver() {}
 };
 
 }  // namespace mus
 }  // namespace ash
 
-#endif  // ASH_MUS_ROOT_WINDOW_OBSERVERS_H_
+#endif  // ASH_MUS_WINDOW_OBSERVER_H_