Implement "pinned" mode in ash.

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 8e4563297def24de6884f563fda109fae1d415d5..0e6b12e20a505a497ff2cd4fb3b3a4c2202584b1 100644
--- a/ash/accelerators/accelerator_controller.h
+++ b/ash/accelerators/accelerator_controller.h
@@ -209,6 +209,8 @@ 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 allowed in pinned mode.
+  std::set<int> actions_allowed_in_pinned_mode_;
   // Actions disallowed if there are no windows.
   std::set<int> actions_needing_window_;
   // Actions that can be performed without closing the menu (if one is present).