ios: Cleanup class/struct forward declarations

ios/chrome/browser/application_context_impl.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_CHROME_BROWSER_APPLICATION_CONTEXT_IMPL_H_
 #define IOS_CHROME_BROWSER_APPLICATION_CONTEXT_IMPL_H_
 
 #include <memory>
 #include <string>
 
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "base/threading/thread_checker.h"
 #include "ios/chrome/browser/application_context.h"
 
-class PrefRegistrySimple;
-
 namespace base {
 class CommandLine;
 class SequencedTaskRunner;
 }
 
 class ApplicationContextImpl : public ApplicationContext {
  public:
   ApplicationContextImpl(base::SequencedTaskRunner* local_state_task_runner,
                          const base::CommandLine& command_line,
                          const std::string& locale);
@@ skipping 63 matching lines @@
   // Sequenced task runner for local state related I/O tasks.
   const scoped_refptr<base::SequencedTaskRunner> local_state_task_runner_;
 
   bool was_last_shutdown_clean_;
   bool created_local_state_;
 
   DISALLOW_COPY_AND_ASSIGN(ApplicationContextImpl);
 };
 
 #endif  // IOS_CHROME_BROWSER_APPLICATION_CONTEXT_IMPL_H_