Make sure manifest specified shortcut for Extension Command can not override the built-in shortcuts.

chrome/browser/ui/cocoa/accelerators_cocoa.h

 // Copyright (c) 2011 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.
 
 #ifndef CHROME_BROWSER_UI_COCOA_ACCELERATORS_COCOA_H_
 #define CHROME_BROWSER_UI_COCOA_ACCELERATORS_COCOA_H_
 
 #include <map>
 
 #include "ui/base/accelerators/accelerator.h"
 
 template <typename T> struct DefaultSingletonTraits;
 
 // This class maintains a map of command_ids to Accelerator objects (see
 // chrome/app/chrome_command_ids.h). Currently, this only lists the commands
 // that are used in the Wrench menu.
 //
 // It is recommended that this class be used as a singleton so that the key map
 // isn't created multiple places.
 //
 //   #import "base/memory/singleton.h"
 //   ...
 //   AcceleratorsCocoa* keymap = AcceleratorsCocoa::GetInstance();
 //   return keymap->GetAcceleratorForCommand(IDC_COPY);
 //
 class AcceleratorsCocoa {
  public:
   typedef std::map<int, ui::Accelerator> AcceleratorMap;
+  typedef AcceleratorMap::const_iterator const_iterator;
+
+  const_iterator const begin() { return accelerators_.begin(); }
+  const_iterator const end() { return accelerators_.end(); }
 
   // Returns NULL if there is no accelerator for the command.
   const ui::Accelerator* GetAcceleratorForCommand(int command_id);
 
   // Returns the singleton instance.
   static AcceleratorsCocoa* GetInstance();
 
  private:
   friend struct DefaultSingletonTraits<AcceleratorsCocoa>;
 
   AcceleratorsCocoa();
   ~AcceleratorsCocoa();
 
   AcceleratorMap accelerators_;
 
   DISALLOW_COPY_AND_ASSIGN(AcceleratorsCocoa);
 };
 
 #endif  // CHROME_BROWSER_UI_COCOA_ACCELERATORS_COCOA_H_