AcceleratorProvider: Make GetAcceleratorForCommandId const.

ui/base/cocoa/menu_controller_unittest.mm

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #import <Cocoa/Cocoa.h>
 
 #include "base/message_loop/message_loop.h"
 #include "base/strings/sys_string_conversions.h"
 #include "base/strings/utf_string_conversions.h"
 #include "third_party/skia/include/core/SkBitmap.h"
@@ skipping 28 matching lines @@
         did_show_(false),
         did_close_(false) {
   }
 
   bool IsCommandIdChecked(int command_id) const override { return false; }
   bool IsCommandIdEnabled(int command_id) const override {
     ++enable_count_;
     return true;
   }
   bool GetAcceleratorForCommandId(int command_id,
-                                  Accelerator* accelerator) override {
+                                  Accelerator* accelerator) const override {
     return false;
   }
   void ExecuteCommand(int command_id, int event_flags) override {
     ++execute_count_;
   }
 
   void MenuWillShow(SimpleMenuModel* /*source*/) override {
     EXPECT_FALSE(did_show_);
     EXPECT_FALSE(did_close_);
     did_show_ = true;
@@ skipping 332 matching lines @@
   // Pump the task that notifies the delegate.
   message_loop.RunUntilIdle();
 
   // Expect that the delegate got notified properly.
   EXPECT_TRUE(delegate.did_close_);
 }
 
 }  // namespace
 
 }  // namespace ui