AcceleratorProvider: Make GetAcceleratorForCommandId const.

ash/shelf/shelf_view_unittest.cc

Index: ash/shelf/shelf_view_unittest.cc
diff --git a/ash/shelf/shelf_view_unittest.cc b/ash/shelf/shelf_view_unittest.cc
index 43eba9dda99a944530645d3d6ff852f14a7d1fd4..90b34883e31017254b158ec52bf9f7147d99ef7c 100644
--- a/ash/shelf/shelf_view_unittest.cc
+++ b/ash/shelf/shelf_view_unittest.cc
@@ -2052,7 +2052,7 @@ class TestShelfMenuModel : public ShelfMenuModel,
     return command_id != 0;
   }
   bool GetAcceleratorForCommandId(int command_id,
-                                  ui::Accelerator* accelerator) override {
+                                  ui::Accelerator* accelerator) const override {
     return false;
   }
   void ExecuteCommand(int command_id, int event_flags) override {}