Remove unused linked_ptr include in extension.h

chrome/browser/background/background_mode_manager.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_BACKGROUND_BACKGROUND_MODE_MANAGER_H_
 #define CHROME_BROWSER_BACKGROUND_BACKGROUND_MODE_MANAGER_H_
 
 #include <map>
 #include <set>
 #include <vector>
 
 #include "base/callback_forward.h"
 #include "base/gtest_prod_util.h"
 #include "base/macros.h"
+#include "base/memory/linked_ptr.h"
 #include "base/memory/scoped_vector.h"
 #include "base/memory/weak_ptr.h"
 #include "chrome/browser/background/background_application_list_model.h"
 #include "chrome/browser/lifetime/scoped_keep_alive.h"
 #include "chrome/browser/profiles/profile_attributes_storage.h"
 #include "chrome/browser/status_icons/status_icon.h"
 #include "chrome/browser/status_icons/status_icon_menu_model.h"
 #include "chrome/browser/ui/browser_list_observer.h"
 #include "components/keyed_service/core/keyed_service.h"
 #include "components/prefs/pref_change_registrar.h"
@@ skipping 436 matching lines @@
 
   // Set to true when background mode is suspended.
   bool background_mode_suspended_;
 
   base::WeakPtrFactory<BackgroundModeManager> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(BackgroundModeManager);
 };
 
 #endif  // CHROME_BROWSER_BACKGROUND_BACKGROUND_MODE_MANAGER_H_