Fix two crash and potential leak&crash

ash/accelerators/accelerator_dispatcher.h

Index: ash/accelerators/accelerator_dispatcher.h
diff --git a/ash/accelerators/accelerator_dispatcher.h b/ash/accelerators/accelerator_dispatcher.h
index af75f48b88aab239e0d468a939a1734c10eae9c7..209b67dcc2ad011902d237c0536b31b2a1fea17e 100644
--- a/ash/accelerators/accelerator_dispatcher.h
+++ b/ash/accelerators/accelerator_dispatcher.h
@@ -9,6 +9,7 @@
 #include "ash/ash_export.h"
 #include "base/message_loop.h"
 #include "ui/aura/window.h"
+#include "ui/aura/window_observer.h"
 
 namespace ash {
 
@@ -16,10 +17,12 @@ namespace ash {
 // Wraps a nested dispatcher to which control is passed if no accelerator key
 // has been pressed.
 // TODO(pkotwicz): Port AcceleratorDispatcher to mac.
-class ASH_EXPORT AcceleratorDispatcher : public MessageLoop::Dispatcher {
+class ASH_EXPORT AcceleratorDispatcher : public MessageLoop::Dispatcher,
+                                         public aura::WindowObserver {
  public:
   explicit AcceleratorDispatcher(MessageLoop::Dispatcher* nested_dispatcher,
                                  aura::Window* associated_window);
+  virtual ~AcceleratorDispatcher();
 
 #if defined(USE_X11)
   virtual base::MessagePumpDispatcher::DispatchStatus Dispatch(
@@ -28,6 +31,9 @@ class ASH_EXPORT AcceleratorDispatcher : public MessageLoop::Dispatcher {
   bool AcceleratorDispatcher::Dispatch(const MSG& msg) OVERRIDE;
 #endif
 
+  // aura::WindowObserver overrides:
+  virtual void OnWindowDestroying(aura::Window* window) OVERRIDE;
+
  private:
   MessageLoop::Dispatcher* nested_dispatcher_;