components: Cleanup class/struct fwd declarations

components/prefs/pref_filter.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 COMPONENTS_PREFS_PREF_FILTER_H_
 #define COMPONENTS_PREFS_PREF_FILTER_H_
 
 #include <memory>
 #include <string>
 #include <utility>
 
 #include "base/callback_forward.h"
 #include "components/prefs/base_prefs_export.h"
 
 namespace base {
 class DictionaryValue;
-class Value;
 }  // namespace base
 
 // Filters preferences as they are loaded from disk or updated at runtime.
 // Currently supported only by JsonPrefStore.
 class COMPONENTS_PREFS_EXPORT PrefFilter {
  public:
   // A pair of pre-write and post-write callbacks.
   using OnWriteCallbackPair =
       std::pair<base::Closure, base::Callback<void(bool success)>>;
 
@@ skipping 27 matching lines @@
   // |pref_store_contents| will be persisted to disk and also affect the
   // in-memory state.
   // If the returned callbacks are non-null, they will be registered to be
   // invoked synchronously after the next write (from the I/O TaskRunner so they
   // must not be bound to thread-unsafe member state).
   virtual OnWriteCallbackPair FilterSerializeData(
       base::DictionaryValue* pref_store_contents) = 0;
 };
 
 #endif  // COMPONENTS_PREFS_PREF_FILTER_H_