extensions: Cleanup class/struct fwd declarations

extensions/shell/browser/shell_browser_main_parts.h

 // Copyright 2014 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 EXTENSIONS_SHELL_BROWSER_SHELL_BROWSER_MAIN_PARTS_H_
 #define EXTENSIONS_SHELL_BROWSER_SHELL_BROWSER_MAIN_PARTS_H_
 
 #include <memory>
 
 #include "base/compiler_specific.h"
 #include "base/macros.h"
 #include "base/task/cancelable_task_tracker.h"
 #include "build/build_config.h"
 #include "content/public/browser/browser_main_parts.h"
 #include "content/public/common/main_function_params.h"
 #include "ui/aura/window_tree_host_observer.h"
 
 class PrefService;
 
 namespace content {
 class BrowserContext;
 struct MainFunctionParams;
 }
 
-namespace views {
-class Widget;
-}
-
 namespace extensions {
 
 class AppWindowClient;
 class DesktopController;
 class ExtensionsBrowserClient;
 class ExtensionsClient;
 class ShellBrowserContext;
 class ShellBrowserMainDelegate;
 class ShellDeviceClient;
 class ShellExtensionSystem;
@@ skipping 68 matching lines @@
 #if !defined(DISABLE_NACL)
   base::CancelableTaskTracker task_tracker_;
 #endif
 
   DISALLOW_COPY_AND_ASSIGN(ShellBrowserMainParts);
 };
 
 }  // namespace extensions
 
 #endif  // EXTENSIONS_SHELL_BROWSER_SHELL_BROWSER_MAIN_PARTS_H_