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

chrome/browser/ui/cocoa/accelerators_cocoa.h

Index: chrome/browser/ui/cocoa/accelerators_cocoa.h
diff --git a/chrome/browser/ui/cocoa/accelerators_cocoa.h b/chrome/browser/ui/cocoa/accelerators_cocoa.h
index b0067afb6fab9bbfe068e22b298448cfe2e5f4b4..cc1840ca5fb18b093e1a96449e2f6ba08c703992 100644
--- a/chrome/browser/ui/cocoa/accelerators_cocoa.h
+++ b/chrome/browser/ui/cocoa/accelerators_cocoa.h
@@ -26,6 +26,10 @@ template <typename T> struct DefaultSingletonTraits;
 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);