Added default implementations of GetAcceleratorForCommandId.

ash/sysui/shelf_delegate_mus.cc

Index: ash/sysui/shelf_delegate_mus.cc
diff --git a/ash/sysui/shelf_delegate_mus.cc b/ash/sysui/shelf_delegate_mus.cc
index d679f56f644c69e6a4f945fefe9ede331a5c099e..6e9b3d3deccecec082f7f017c64f5aebba9bdbbf 100644
--- a/ash/sysui/shelf_delegate_mus.cc
+++ b/ash/sysui/shelf_delegate_mus.cc
@@ -88,11 +88,6 @@ class ShelfItemDelegateMus : public ShelfItemDelegate {
     bool IsCommandIdEnabled(int command_id) const override {
       return command_id > 0;
     }
-    bool GetAcceleratorForCommandId(
-        int command_id,
-        ui::Accelerator* accelerator) const override {
-      return false;
-    }
     void ExecuteCommand(int command_id, int event_flags) override {
       item_delegate_->user_window_controller_->ActivateUserWindow(command_id);
     }