chrome: Extract CommandUpdaterDelegate into its own header file.

chrome/browser/command_observer.h

 // Copyright (c) 2012 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_COMMAND_OBSERVER_H_
 #define CHROME_BROWSER_COMMAND_OBSERVER_H_
 
 // An Observer interface implemented by objects that want to be informed when
 // the state of a particular command ID is modified. See CommandUpdater.
 class CommandObserver {
  public:
   // Notifies the observer that the enabled state has changed for the
   // specified command id.
   virtual void EnabledStateChangedForCommand(int id, bool enabled) = 0;
 
  protected:
-  virtual ~CommandObserver();
+  virtual ~CommandObserver() {}
 };
 
 #endif  // CHROME_BROWSER_COMMAND_OBSERVER_H_