components: Cleanup class/struct fwd declarations

components/user_prefs/tracked/tracked_preferences_migration.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 COMPONENTS_USER_PREFS_TRACKED_TRACKED_PREFERENCES_MIGRATION_H_
 #define COMPONENTS_USER_PREFS_TRACKED_TRACKED_PREFERENCES_MIGRATION_H_
 
 #include <memory>
 #include <set>
 #include <string>
 
 #include "base/callback_forward.h"
 #include "components/user_prefs/tracked/pref_hash_store.h"
 
-class HashStoreContents;
 class InterceptablePrefFilter;
 class PrefHashStore;
 
 // Sets up InterceptablePrefFilter::FilterOnLoadInterceptors on
 // |unprotected_pref_filter| and |protected_pref_filter| which prevents each
 // filter from running their on load operations until the interceptors decide to
 // hand the prefs back to them (after migration is complete). |
 // (un)protected_store_cleaner| and
 // |register_on_successful_(un)protected_store_write_callback| are used to do
 // post-migration cleanup tasks. Those should be bound to weak pointers to avoid
@@ skipping 11 matching lines @@
     const base::Callback<void(const base::Closure&)>&
         register_on_successful_unprotected_store_write_callback,
     const base::Callback<void(const base::Closure&)>&
         register_on_successful_protected_store_write_callback,
     std::unique_ptr<PrefHashStore> unprotected_pref_hash_store,
     std::unique_ptr<PrefHashStore> protected_pref_hash_store,
     InterceptablePrefFilter* unprotected_pref_filter,
     InterceptablePrefFilter* protected_pref_filter);
 
 #endif  // COMPONENTS_USER_PREFS_TRACKED_TRACKED_PREFERENCES_MIGRATION_H_