Fix two crash and potential leak&crash

ash/accelerators/accelerator_dispatcher.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ash/accelerators/accelerator_dispatcher.h"
 
 namespace ash {
 
 AcceleratorDispatcher::AcceleratorDispatcher(
     MessageLoop::Dispatcher* nested_dispatcher, aura::Window* associated_window)
     : nested_dispatcher_(nested_dispatcher),
       associated_window_(associated_window) {
   DCHECK(nested_dispatcher_);
-  DCHECK(associated_window_);
+  associated_window_->AddObserver(this);
+}
+
+AcceleratorDispatcher::~AcceleratorDispatcher() {
+  if (associated_window_)
+    associated_window_->RemoveObserver(this);
+}
+
+void AcceleratorDispatcher::OnWindowDestroying(aura::Window* window) {
+  if (associated_window_ == window)
+    associated_window_ = NULL;
 }
 
 }  // namespace ash