Fixes shutdown race in mash

ash/mus/accelerator_registrar_impl.h

Index: ash/mus/accelerator_registrar_impl.h
diff --git a/ash/mus/accelerator_registrar_impl.h b/ash/mus/accelerator_registrar_impl.h
index e720674e1dd1a7b0f63ce7d5142a988a94d89d1e..1985a747802f5ce0f5dd36d0ae3b37af6b1dff78 100644
--- a/ash/mus/accelerator_registrar_impl.h
+++ b/ash/mus/accelerator_registrar_impl.h
@@ -9,6 +9,7 @@
 
 #include <map>
 
+#include "ash/mus/window_manager_observer.h"
 #include "base/callback.h"
 #include "base/macros.h"
 #include "components/mus/public/interfaces/accelerator_registrar.mojom.h"
@@ -24,7 +25,8 @@ class WindowManager;
 // connection. This manages its own lifetime, and destroys itself when the
 // AcceleratorRegistrar and all its AcceleratorHandlers are disconnected. Upon
 // destruction, it calls the DestroyCallback.
-class AcceleratorRegistrarImpl : public ::mus::mojom::AcceleratorRegistrar {
+class AcceleratorRegistrarImpl : public ::mus::mojom::AcceleratorRegistrar,
+                                 public WindowManagerObserver {
  public:
   using DestroyCallback = base::Callback<void(AcceleratorRegistrarImpl*)>;
   AcceleratorRegistrarImpl(WindowManager* window_manager,
@@ -56,6 +58,10 @@ class AcceleratorRegistrarImpl : public ::mus::mojom::AcceleratorRegistrar {
                       const AddAcceleratorCallback& callback) override;
   void RemoveAccelerator(uint32_t accelerator_id) override;
 
+  // WindowManagerObserver:
+  void OnAccelerator(uint32_t id, const ui::Event& event) override;
+  void OnWindowTreeClientDestroyed() override;
+
   WindowManager* window_manager_;
   ::mus::mojom::AcceleratorHandlerPtr accelerator_handler_;
   mojo::Binding<AcceleratorRegistrar> binding_;