Use the default dispatcher where possible for nested message loops.

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 a7ae40aaf84ca6932f317b18a7cbfed6665266f5..a46368cfb45420674ddbb41bc25922cd96c0a52a 100644
--- a/ash/accelerators/accelerator_dispatcher.h
+++ b/ash/accelerators/accelerator_dispatcher.h
@@ -15,8 +15,8 @@ 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,