chrome: Cleanup class/struct forward declarations

chrome/browser/sync_file_system/drive_backend/metadata_database_index_on_disk.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 CHROME_BROWSER_SYNC_FILE_SYSTEM_DRIVE_BACKEND_METADATA_DATABASE_INDEX_ON_DISK_H_
 #define CHROME_BROWSER_SYNC_FILE_SYSTEM_DRIVE_BACKEND_METADATA_DATABASE_INDEX_ON_DISK_H_
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <map>
 #include <set>
 #include <string>
 #include <vector>
 
 #include "base/macros.h"
 #include "chrome/browser/sync_file_system/drive_backend/metadata_database_index_interface.h"
 #include "chrome/browser/sync_file_system/drive_backend/tracker_id_set.h"
 
 namespace sync_file_system {
 namespace drive_backend {
 
 class FileMetadata;
 class FileTracker;
 class LevelDBWrapper;
 class ServiceMetadata;
-struct DatabaseContents;
 // TODO(peria): Migrate implementation of ParentIDAndTitle structure from
 //     metadata_database_index.{cc,h} to here, on removing the files.
 struct ParentIDAndTitle;
 
 // Maintains indexes of MetadataDatabase on disk.
 class MetadataDatabaseIndexOnDisk : public MetadataDatabaseIndexInterface {
  public:
   static std::unique_ptr<MetadataDatabaseIndexOnDisk> Create(
       LevelDBWrapper* db);
 
@@ skipping 134 matching lines @@
 
   size_t num_dirty_trackers_;
 
   DISALLOW_COPY_AND_ASSIGN(MetadataDatabaseIndexOnDisk);
 };
 
 }  // namespace drive_backend
 }  // namespace sync_file_system
 
 #endif  // CHROME_BROWSER_SYNC_FILE_SYSTEM_DRIVE_BACKEND_METADATA_DATABASE_INDEX_ON_DISK_H_