ios: Cleanup class/struct forward declarations

ios/web/shell/shell_browser_state.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 IOS_WEB_SHELL_SHELL_BROWSER_STATE_H_
 #define IOS_WEB_SHELL_SHELL_BROWSER_STATE_H_
 
 #include "base/files/file_path.h"
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "ios/web/public/browser_state.h"
 
 namespace web {
 
-class ShellBrowserContext;
 class ShellURLRequestContextGetter;
 
 // Shell-specific implementation of BrowserState.  Can only be called from the
 // UI thread.
 class ShellBrowserState : public BrowserState {
  public:
   ShellBrowserState();
   ~ShellBrowserState() override;
 
   // BrowserState implementation.
   bool IsOffTheRecord() const override;
   base::FilePath GetStatePath() const override;
   net::URLRequestContextGetter* GetRequestContext() override;
 
  private:
   base::FilePath path_;
   scoped_refptr<ShellURLRequestContextGetter> request_context_getter_;
 
   DISALLOW_COPY_AND_ASSIGN(ShellBrowserState);
 };
 
 }  // namespace web
 
 #endif  // IOS_WEB_SHELL_SHELL_BROWSER_STATE_H_