Use the default dispatcher where possible for nested message loops.

ash/accelerators/accelerator_dispatcher.h

 // 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.
 
 #ifndef ASH_ACCELERATORS_ACCELERATOR_DISPATCHER_H_
 #define ASH_ACCELERATORS_ACCELERATOR_DISPATCHER_H_
 
 #include "ash/ash_export.h"
 #include "base/message_loop/message_pump_dispatcher.h"
 #include "ui/aura/window.h"
 #include "ui/aura/window_observer.h"
 
 namespace ash {
 
 // Dispatcher for handling accelerators from menu.
 //
 // Wraps a nested dispatcher to which control is passed if no accelerator key
-// has been pressed.
-// TODO(pkotwicz): Port AcceleratorDispatcher to mac.
+// has been pressed. If the nested dispatcher is NULL, then the control is
+// passed back to the default dispatcher.
 // TODO(pkotwicz): Add support for a |nested_dispatcher| which sends
 //  events to a system IME.
 class ASH_EXPORT AcceleratorDispatcher : public base::MessagePumpDispatcher,
                                          public aura::WindowObserver {
  public:
   AcceleratorDispatcher(base::MessagePumpDispatcher* nested_dispatcher,
                         aura::Window* associated_window);
   virtual ~AcceleratorDispatcher();
 
   // MessagePumpDispatcher overrides:
   virtual uint32_t Dispatch(const base::NativeEvent& event) OVERRIDE;
 
   // aura::WindowObserver overrides:
   virtual void OnWindowDestroying(aura::Window* window) OVERRIDE;
 
  private:
   base::MessagePumpDispatcher* nested_dispatcher_;
 
   // Window associated with |nested_dispatcher_| which is used to determine
   // whether the |nested_dispatcher_| is allowed to receive events.
   aura::Window* associated_window_;
 
   DISALLOW_COPY_AND_ASSIGN(AcceleratorDispatcher);
 };
 
 }  // namespace ash
 
 #endif  // ASH_ACCELERATORS_ACCELERATOR_DISPATCHER_H_