For Mac, add AppListViewDelegateObserver::OnShutdown

ui/app_list/app_list_view_delegate_observer.h

 // 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.
 
 #ifndef UI_APP_LIST_APP_LIST_VIEW_DELEGATE_OBSERVER_H_
 #define UI_APP_LIST_APP_LIST_VIEW_DELEGATE_OBSERVER_H_
 
 #include "ui/app_list/app_list_export.h"
 
 namespace app_list {
 
 class APP_LIST_EXPORT AppListViewDelegateObserver {
  public:
   // Invoked when the Profiles shown on the app list change, or the active
   // profile changes its signin status.
   virtual void OnProfilesChanged() = 0;
 
+  // Invoked on Chrome shutdown. This is only needed on Mac, since reference-
+  // counting in Objective-C means that simply closing the window isn't enough
+  // to guarantee references to Chrome objects are gone.
+  virtual void OnShutdown() = 0;
+
  protected:
   virtual ~AppListViewDelegateObserver() {}
 };
 
 }  // namespace app_list
 
 #endif  // UI_APP_LIST_APP_LIST_VIEW_DELEGATE_OBSERVER_H_