ash: Cleanup class/struct forward declarations

ash/shell/content/client/shell_browser_main_parts.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 ASH_SHELL_CONTENT_CLIENT_EXAMPLES_BROWSER_MAIN_PARTS_H_
 #define ASH_SHELL_CONTENT_CLIENT_EXAMPLES_BROWSER_MAIN_PARTS_H_
 
 #include <memory>
 
 #include "base/macros.h"
 #include "content/public/browser/browser_main_parts.h"
 
-namespace base {
-class Thread;
-}
-
 namespace content {
 class ShellBrowserContext;
 struct MainFunctionParams;
 }
 
 namespace net {
 class NetLog;
 }
 
 namespace views {
@@ skipping 35 matching lines @@
   ShellDelegateImpl* delegate_;  // owned by Shell
   std::unique_ptr<wm::WMState> wm_state_;
 
   DISALLOW_COPY_AND_ASSIGN(ShellBrowserMainParts);
 };
 
 }  // namespace shell
 }  // namespace ash
 
 #endif  // ASH_SHELL_CONTENT_CLIENT_EXAMPLES_BROWSER_MAIN_PARTS_H_