Use the default dispatcher where possible for nested message loops.

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"
 
 #if defined(USE_X11)
 #include <X11/Xlib.h>
 
 // Xlib defines RootWindow
@@ skipping 52 matching lines @@
   return false;
 }
 
 }  // namespace
 
 AcceleratorDispatcher::AcceleratorDispatcher(
     base::MessagePumpDispatcher* nested_dispatcher,
     aura::Window* associated_window)
     : nested_dispatcher_(nested_dispatcher),
       associated_window_(associated_window) {
-  DCHECK(nested_dispatcher_);
   associated_window_->AddObserver(this);
 }
 
 AcceleratorDispatcher::~AcceleratorDispatcher() {
   if (associated_window_)
     associated_window_->RemoveObserver(this);
 }
 
 void AcceleratorDispatcher::OnWindowDestroying(aura::Window* window) {
   if (associated_window_ == window)
@@ skipping 30 matching lines @@
       if (key_event.type() == ui::ET_KEY_RELEASED)
         accelerator.set_type(ui::ET_KEY_RELEASED);
       // Fill out context object so AcceleratorController will know what
       // was the previous accelerator or if the current accelerator is repeated.
       Shell::GetInstance()->accelerator_controller()->context()->
           UpdateContext(accelerator);
       if (accelerator_controller->Process(accelerator))
         return POST_DISPATCH_NONE;
     }
 
-    return nested_dispatcher_->Dispatch(key_event.native_event());
+    return nested_dispatcher_
+               ? nested_dispatcher_->Dispatch(key_event.native_event())
+               : POST_DISPATCH_PERFORM_DEFAULT;
   }
 
-  return nested_dispatcher_->Dispatch(event);
+  return nested_dispatcher_ ? nested_dispatcher_->Dispatch(event)
+                            : POST_DISPATCH_PERFORM_DEFAULT;
 }
 
 }  // namespace ash