Use the default dispatcher where possible for nested message loops.

ash/accelerators/nested_dispatcher_controller.cc

Index: ash/accelerators/nested_dispatcher_controller.cc
diff --git a/ash/accelerators/nested_dispatcher_controller.cc b/ash/accelerators/nested_dispatcher_controller.cc
index c551e948a5f0a59fb10184e319dbb53bd1cf5436..6977f07952ed5a8f08dc9596d94c01fc32b3a00a 100644
--- a/ash/accelerators/nested_dispatcher_controller.cc
+++ b/ash/accelerators/nested_dispatcher_controller.cc
@@ -6,6 +6,7 @@
 
 #include "ash/accelerators/accelerator_dispatcher.h"
 #include "ash/shell.h"
+#include "base/auto_reset.h"
 #include "base/run_loop.h"
 
 namespace ash {
@@ -27,7 +28,14 @@ void NestedDispatcherController::RunWithDispatcher(
   // TODO(jbates) crbug.com/134753 Find quitters of this RunLoop and have them
   //              use run_loop.QuitClosure().
   base::RunLoop run_loop(&dispatcher);
+  base::AutoReset<base::Closure> reset_closure(&quit_closure_,
+                                               run_loop.QuitClosure());
   run_loop.Run();
 }
 
+void NestedDispatcherController::QuitNestedMessageLoop() {
+  CHECK(!quit_closure_.is_null());
+  quit_closure_.Run();
+}
+
 }  // namespace ash