Use the default dispatcher where possible for nested message loops.

ash/accelerators/nested_dispatcher_controller.h

Index: ash/accelerators/nested_dispatcher_controller.h
diff --git a/ash/accelerators/nested_dispatcher_controller.h b/ash/accelerators/nested_dispatcher_controller.h
index 958cbae26da5a611bdecacb4d89c5466a3f6fa5a..1f8b52e64394c98039129204eb4c54376b35f48e 100644
--- a/ash/accelerators/nested_dispatcher_controller.h
+++ b/ash/accelerators/nested_dispatcher_controller.h
@@ -6,6 +6,7 @@
 #define ASH_ACCELERATORS_NESTED_DISPATCHER_CONTROLLER_H_
 
 #include "ash/ash_export.h"
+#include "base/callback.h"
 #include "base/message_loop/message_loop.h"
 #include "ui/aura/client/dispatcher_client.h"
 #include "ui/aura/window.h"
@@ -22,10 +23,14 @@ class ASH_EXPORT NestedDispatcherController
   NestedDispatcherController();
   virtual ~NestedDispatcherController();
 
+  // aura::client::DispatcherClient:
   virtual void RunWithDispatcher(base::MessagePumpDispatcher* dispatcher,
                                  aura::Window* associated_window) OVERRIDE;
+  virtual void QuitNestedMessageLoop() OVERRIDE;
 
  private:
+  base::Closure quit_closure_;
+
   DISALLOW_COPY_AND_ASSIGN(NestedDispatcherController);
 };