extensions: Cleanup class/struct fwd declarations

extensions/browser/value_store/leveldb_value_store.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 EXTENSIONS_BROWSER_VALUE_STORE_LEVELDB_VALUE_STORE_H_
 #define EXTENSIONS_BROWSER_VALUE_STORE_LEVELDB_VALUE_STORE_H_
 
 #include <stddef.h>
 
 #include <memory>
 #include <string>
 #include <vector>
 
 #include "base/compiler_specific.h"
 #include "base/files/file_path.h"
 #include "base/macros.h"
 #include "base/trace_event/memory_dump_provider.h"
 #include "extensions/browser/value_store/lazy_leveldb.h"
 #include "extensions/browser/value_store/value_store.h"
 #include "third_party/leveldatabase/src/include/leveldb/db.h"
 
-namespace base {
-class HistogramBase;
-}  // namespace base
-
 // Value store area, backed by a leveldb database.
 // All methods must be run on the FILE thread.
 class LeveldbValueStore : public ValueStore,
                           public LazyLevelDb,
                           public base::trace_event::MemoryDumpProvider {
  public:
   // Creates a database bound to |path|. The underlying database won't be
   // opened (i.e. may not be created) until one of the get/set/etc methods are
   // called - this is because opening the database may fail, and extensions
   // need to be notified of that, but we don't want to permanently give up.
@@ skipping 38 matching lines @@
                                 leveldb::WriteBatch* batch,
                                 ValueStoreChangeList* changes);
 
   // Commits the changes in |batch| to the database.
   ValueStore::Status WriteToDb(leveldb::WriteBatch* batch);
 
   DISALLOW_COPY_AND_ASSIGN(LeveldbValueStore);
 };
 
 #endif  // EXTENSIONS_BROWSER_VALUE_STORE_LEVELDB_VALUE_STORE_H_