Add an accessibility alert for incorrect use of some accelerators.

ash/accelerators/accelerator_controller.h

Index: ash/accelerators/accelerator_controller.h
diff --git a/ash/accelerators/accelerator_controller.h b/ash/accelerators/accelerator_controller.h
index cacba0e0b21acf723eda46516d7adff1c7ad9c2f..5689a419e5f6d20cf54f34a19d5ab3357618c307 100644
--- a/ash/accelerators/accelerator_controller.h
+++ b/ash/accelerators/accelerator_controller.h
@@ -169,6 +169,8 @@ class ASH_EXPORT AcceleratorController : public ui::AcceleratorTarget {
   std::set<int> nonrepeatable_actions_;
   // Actions allowed in app mode.
   std::set<int> actions_allowed_in_app_mode_;
+  // Actions disallowed if there are no windows.
+  std::set<int> actions_needing_window_;
 
   DISALLOW_COPY_AND_ASSIGN(AcceleratorController);
 };